diff --git a/blocks/api/parser.js b/blocks/api/parser.js
index 63ad01f98ab358..63af6229e39912 100644
--- a/blocks/api/parser.js
+++ b/blocks/api/parser.js
@@ -10,6 +10,7 @@ import { pickBy } from 'lodash';
 import { parse as grammarParse } from './post.pegjs';
 import { getBlockType, getUnknownTypeHandler } from './registration';
 import { createBlock } from './factory';
+import { getBeautifulContent, getSaveContent } from './serializer';
 
 /**
  * Returns the block attributes parsed from raw content.
@@ -81,18 +82,59 @@ export function createBlockWithFallback( name, rawContent, attributes ) {
 
 	// Include in set only if type were determined.
 	// TODO do we ever expect there to not be an unknown type handler?
-	if ( blockType && ( rawContent.trim() || name !== fallbackBlock ) ) {
+	if ( blockType && ( rawContent || name !== fallbackBlock ) ) {
 		// TODO allow blocks to opt-in to receiving a tree instead of a string.
 		// Gradually convert all blocks to this new format, then remove the
 		// string serialization.
 		const block = createBlock(
 			name,
-			getBlockAttributes( blockType, rawContent.trim(), attributes )
+			getBlockAttributes( blockType, rawContent, attributes )
 		);
+
+		// Validate that the parsed block is valid, meaning that if we were to
+		// reserialize it given the assumed attributes, the markup matches the
+		// original value. Otherwise, preserve original to avoid destruction.
+		block.isValid = isValidBlock( rawContent, blockType, block.attributes );
+		if ( ! block.isValid ) {
+			block.originalContent = rawContent;
+		}
+
 		return block;
 	}
 }
 
+/**
+ * Returns true if the parsed block is valid given the input content. A block
+ * is considered valid if, when serialized with assumed attributes, the content
+ * matches the original value.
+ *
+ * Logs to console in development environments when invalid.
+ *
+ * @param  {String}  rawContent Original block content
+ * @param  {String}  blockType  Block type
+ * @param  {Object}  attributes Parsed block attributes
+ * @return {Boolean}            Whether block is valid
+ */
+export function isValidBlock( rawContent, blockType, attributes ) {
+	const [ actual, expected ] = [
+		rawContent,
+		getSaveContent( blockType, attributes ),
+	].map( getBeautifulContent );
+
+	const isValid = ( actual === expected );
+
+	if ( ! isValid && 'development' === process.env.NODE_ENV ) {
+		// eslint-disable-next-line no-console
+		console.error(
+			'Invalid block parse\n' +
+				'\tExpected: ' + expected + '\n' +
+				'\tActual:   ' + actual
+		);
+	}
+
+	return isValid;
+}
+
 /**
  * Parses the post content with a PegJS grammar and returns a list of blocks.
  *
@@ -102,7 +144,7 @@ export function createBlockWithFallback( name, rawContent, attributes ) {
 export function parseWithGrammar( content ) {
 	return grammarParse( content ).reduce( ( memo, blockNode ) => {
 		const { blockName, rawContent, attrs } = blockNode;
-		const block = createBlockWithFallback( blockName, rawContent, attrs );
+		const block = createBlockWithFallback( blockName, rawContent.trim(), attrs );
 		if ( block ) {
 			memo.push( block );
 		}
diff --git a/blocks/api/serializer.js b/blocks/api/serializer.js
index aed8cb12f3c88e..2657e727c30f56 100644
--- a/blocks/api/serializer.js
+++ b/blocks/api/serializer.js
@@ -111,10 +111,33 @@ export function serializeAttributes( attrs ) {
 		.replace( /&/g, '\\u0026' ); // ibid
 }
 
+/**
+ * Returns HTML markup processed by a markup beautifier configured for use in
+ * block serialization.
+ *
+ * @param  {String} content Original HTML
+ * @return {String}         Beautiful HTML
+ */
+export function getBeautifulContent( content ) {
+	return beautifyHtml( content, {
+		indent_inner_html: true,
+		wrap_line_length: 0,
+	} );
+}
+
 export function serializeBlock( block ) {
 	const blockName = block.name;
 	const blockType = getBlockType( blockName );
-	const saveContent = getSaveContent( blockType, block.attributes );
+
+	let saveContent;
+	if ( block.isValid ) {
+		saveContent = getSaveContent( blockType, block.attributes );
+	} else {
+		// If block was parsed as invalid, skip serialization behavior and opt
+		// to use original content instead so we don't destroy user content.
+		saveContent = block.originalContent;
+	}
+
 	const saveAttributes = getCommentAttributes( block.attributes, parseBlockAttributes( saveContent, blockType.attributes ) );
 
 	if ( 'wp:core/more' === blockName ) {
@@ -131,13 +154,7 @@ export function serializeBlock( block ) {
 
 	return (
 		`<!-- wp:${ blockName } ${ serializedAttributes }-->\n` +
-
-		/** make more readable - @see https://github.com/WordPress/gutenberg/pull/663 */
-		beautifyHtml( saveContent, {
-			indent_inner_html: true,
-			wrap_line_length: 0,
-		} ) +
-
+		getBeautifulContent( saveContent ) +
 		`\n<!-- /wp:${ blockName } -->`
 	);
 }
diff --git a/blocks/api/test/parser.js b/blocks/api/test/parser.js
index 4d2ba513181800..17632802137d79 100644
--- a/blocks/api/test/parser.js
+++ b/blocks/api/test/parser.js
@@ -10,6 +10,7 @@ import { text } from '../query';
 import {
 	getBlockAttributes,
 	parseBlockAttributes,
+	isValidBlock,
 	createBlockWithFallback,
 	default as parse,
 } from '../parser';
@@ -17,11 +18,14 @@ import {
 	registerBlockType,
 	unregisterBlockType,
 	getBlockTypes,
+	getBlockType,
 	setUnknownTypeHandler,
 } from '../registration';
 
 describe( 'block parser', () => {
-	const defaultBlockSettings = { save: noop };
+	const defaultBlockSettings = {
+		save: ( { attributes } ) => attributes.fruit,
+	};
 
 	afterEach( () => {
 		setUnknownTypeHandler( undefined );
@@ -89,6 +93,28 @@ describe( 'block parser', () => {
 		} );
 	} );
 
+	describe( 'isValidBlock()', () => {
+		it( 'returns false is block is not valid', () => {
+			registerBlockType( 'core/test-block', defaultBlockSettings );
+
+			expect( isValidBlock(
+				'Apples',
+				getBlockType( 'core/test-block' ),
+				{ fruit: 'Bananas' }
+			) ).toBe( false );
+		} );
+
+		it( 'returns true is block is valid', () => {
+			registerBlockType( 'core/test-block', defaultBlockSettings );
+
+			expect( isValidBlock(
+				'Bananas',
+				getBlockType( 'core/test-block' ),
+				{ fruit: 'Bananas' }
+			) ).toBe( true );
+		} );
+	} );
+
 	describe( 'createBlockWithFallback', () => {
 		it( 'should create the requested block if it exists', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
diff --git a/blocks/api/test/serializer.js b/blocks/api/test/serializer.js
index 73705b0846c108..97dda9b7ea30a3 100644
--- a/blocks/api/test/serializer.js
+++ b/blocks/api/test/serializer.js
@@ -6,7 +6,12 @@ import { createElement, Component } from 'element';
 /**
  * Internal dependencies
  */
-import serialize, { getCommentAttributes, getSaveContent, serializeAttributes } from '../serializer';
+import serialize, {
+	getCommentAttributes,
+	getBeautifulContent,
+	getSaveContent,
+	serializeAttributes,
+} from '../serializer';
 import { getBlockTypes, registerBlockType, unregisterBlockType } from '../registration';
 
 describe( 'block serializer', () => {
@@ -16,6 +21,14 @@ describe( 'block serializer', () => {
 		} );
 	} );
 
+	describe( 'getBeautifulContent()', () => {
+		it( 'returns beautiful content', () => {
+			const content = getBeautifulContent( '<div><div>Beautiful</div></div>' );
+
+			expect( content ).toBe( '<div>\n    <div>Beautiful</div>\n</div>' );
+		} );
+	} );
+
 	describe( 'getSaveContent()', () => {
 		describe( 'function save', () => {
 			it( 'should return string verbatim', () => {
@@ -185,6 +198,7 @@ describe( 'block serializer', () => {
 						content: 'Ribs & Chicken',
 						stuff: 'left & right -- but <not>',
 					},
+					isValid: true,
 				},
 			];
 			const expectedPostContent = '<!-- wp:core/test-block {"stuff":"left \\u0026 right \\u002d\\u002d but \\u003cnot\\u003e"} -->\n<p class="wp-block-test-block">Ribs & Chicken</p>\n<!-- /wp:core/test-block -->';
diff --git a/blocks/test/fixtures/core-embed__animoto.json b/blocks/test/fixtures/core-embed__animoto.json
index 28c7c26cd2cc2a..71eac6be8e7561 100644
--- a/blocks/test/fixtures/core-embed__animoto.json
+++ b/blocks/test/fixtures/core-embed__animoto.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from animoto"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__animoto.serialized.html b/blocks/test/fixtures/core-embed__animoto.serialized.html
index e6af9836343d1e..2d00b4dc7b234c 100644
--- a/blocks/test/fixtures/core-embed__animoto.serialized.html
+++ b/blocks/test/fixtures/core-embed__animoto.serialized.html
@@ -3,4 +3,4 @@
     https://animoto.com/
     <figcaption>Embedded content from animoto</figcaption>
 </figure>
-<!-- /wp:core-embed/animoto -->
+<!-- /wp:core-embed/animoto -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__cloudup.json b/blocks/test/fixtures/core-embed__cloudup.json
index 22a55a4d296f99..5d5041cae3fdae 100644
--- a/blocks/test/fixtures/core-embed__cloudup.json
+++ b/blocks/test/fixtures/core-embed__cloudup.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from cloudup"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__cloudup.serialized.html b/blocks/test/fixtures/core-embed__cloudup.serialized.html
index 48f824b9c95cfa..8317513dabbaea 100644
--- a/blocks/test/fixtures/core-embed__cloudup.serialized.html
+++ b/blocks/test/fixtures/core-embed__cloudup.serialized.html
@@ -3,4 +3,4 @@
     https://cloudup.com/
     <figcaption>Embedded content from cloudup</figcaption>
 </figure>
-<!-- /wp:core-embed/cloudup -->
+<!-- /wp:core-embed/cloudup -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__collegehumor.json b/blocks/test/fixtures/core-embed__collegehumor.json
index 47cff93865c5fa..710ee963585829 100644
--- a/blocks/test/fixtures/core-embed__collegehumor.json
+++ b/blocks/test/fixtures/core-embed__collegehumor.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from collegehumor"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__collegehumor.serialized.html b/blocks/test/fixtures/core-embed__collegehumor.serialized.html
index e6870f568d0a78..e55060027e7e63 100644
--- a/blocks/test/fixtures/core-embed__collegehumor.serialized.html
+++ b/blocks/test/fixtures/core-embed__collegehumor.serialized.html
@@ -3,4 +3,4 @@
     https://collegehumor.com/
     <figcaption>Embedded content from collegehumor</figcaption>
 </figure>
-<!-- /wp:core-embed/collegehumor -->
+<!-- /wp:core-embed/collegehumor -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__dailymotion.json b/blocks/test/fixtures/core-embed__dailymotion.json
index 4ad16679bf20e9..90d547507833fa 100644
--- a/blocks/test/fixtures/core-embed__dailymotion.json
+++ b/blocks/test/fixtures/core-embed__dailymotion.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from dailymotion"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__dailymotion.serialized.html b/blocks/test/fixtures/core-embed__dailymotion.serialized.html
index 1cf35255677705..17a3680bf33bc1 100644
--- a/blocks/test/fixtures/core-embed__dailymotion.serialized.html
+++ b/blocks/test/fixtures/core-embed__dailymotion.serialized.html
@@ -3,4 +3,4 @@
     https://dailymotion.com/
     <figcaption>Embedded content from dailymotion</figcaption>
 </figure>
-<!-- /wp:core-embed/dailymotion -->
+<!-- /wp:core-embed/dailymotion -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__facebook.json b/blocks/test/fixtures/core-embed__facebook.json
index 07e08896c62b42..3b91660cfbd3a2 100644
--- a/blocks/test/fixtures/core-embed__facebook.json
+++ b/blocks/test/fixtures/core-embed__facebook.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from facebook"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__facebook.serialized.html b/blocks/test/fixtures/core-embed__facebook.serialized.html
index a2dd1ce7dd9e9b..b0134a99a6a295 100644
--- a/blocks/test/fixtures/core-embed__facebook.serialized.html
+++ b/blocks/test/fixtures/core-embed__facebook.serialized.html
@@ -3,4 +3,4 @@
     https://facebook.com/
     <figcaption>Embedded content from facebook</figcaption>
 </figure>
-<!-- /wp:core-embed/facebook -->
+<!-- /wp:core-embed/facebook -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__flickr.json b/blocks/test/fixtures/core-embed__flickr.json
index 35c28ec6b70e2f..2b264a5aff0e2a 100644
--- a/blocks/test/fixtures/core-embed__flickr.json
+++ b/blocks/test/fixtures/core-embed__flickr.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from flickr"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__flickr.serialized.html b/blocks/test/fixtures/core-embed__flickr.serialized.html
index b6d133afc5d35f..060005490065c3 100644
--- a/blocks/test/fixtures/core-embed__flickr.serialized.html
+++ b/blocks/test/fixtures/core-embed__flickr.serialized.html
@@ -3,4 +3,4 @@
     https://flickr.com/
     <figcaption>Embedded content from flickr</figcaption>
 </figure>
-<!-- /wp:core-embed/flickr -->
+<!-- /wp:core-embed/flickr -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__funnyordie.json b/blocks/test/fixtures/core-embed__funnyordie.json
index 575ed6342f43b6..24907395f8f526 100644
--- a/blocks/test/fixtures/core-embed__funnyordie.json
+++ b/blocks/test/fixtures/core-embed__funnyordie.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from funnyordie"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__funnyordie.serialized.html b/blocks/test/fixtures/core-embed__funnyordie.serialized.html
index 21b1f2bf4aacfe..5d6cf0e352f29f 100644
--- a/blocks/test/fixtures/core-embed__funnyordie.serialized.html
+++ b/blocks/test/fixtures/core-embed__funnyordie.serialized.html
@@ -3,4 +3,4 @@
     https://funnyordie.com/
     <figcaption>Embedded content from funnyordie</figcaption>
 </figure>
-<!-- /wp:core-embed/funnyordie -->
+<!-- /wp:core-embed/funnyordie -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__hulu.json b/blocks/test/fixtures/core-embed__hulu.json
index 35baceb73b1c0c..d2c9744e06a946 100644
--- a/blocks/test/fixtures/core-embed__hulu.json
+++ b/blocks/test/fixtures/core-embed__hulu.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from hulu"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__hulu.serialized.html b/blocks/test/fixtures/core-embed__hulu.serialized.html
index 29a851edcdd0b3..54fed29b9abf03 100644
--- a/blocks/test/fixtures/core-embed__hulu.serialized.html
+++ b/blocks/test/fixtures/core-embed__hulu.serialized.html
@@ -3,4 +3,4 @@
     https://hulu.com/
     <figcaption>Embedded content from hulu</figcaption>
 </figure>
-<!-- /wp:core-embed/hulu -->
+<!-- /wp:core-embed/hulu -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__imgur.json b/blocks/test/fixtures/core-embed__imgur.json
index ea3b38321bec05..7ad645a4371c89 100644
--- a/blocks/test/fixtures/core-embed__imgur.json
+++ b/blocks/test/fixtures/core-embed__imgur.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from imgur"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__imgur.serialized.html b/blocks/test/fixtures/core-embed__imgur.serialized.html
index 9a05b120d43947..982e490a212f43 100644
--- a/blocks/test/fixtures/core-embed__imgur.serialized.html
+++ b/blocks/test/fixtures/core-embed__imgur.serialized.html
@@ -3,4 +3,4 @@
     https://imgur.com/
     <figcaption>Embedded content from imgur</figcaption>
 </figure>
-<!-- /wp:core-embed/imgur -->
+<!-- /wp:core-embed/imgur -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__instagram.json b/blocks/test/fixtures/core-embed__instagram.json
index b47836c1c0daa2..9ddaf1fbaf6306 100644
--- a/blocks/test/fixtures/core-embed__instagram.json
+++ b/blocks/test/fixtures/core-embed__instagram.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from instagram"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__instagram.serialized.html b/blocks/test/fixtures/core-embed__instagram.serialized.html
index bb2c9f04c0dbc8..cbbac9624ab3ea 100644
--- a/blocks/test/fixtures/core-embed__instagram.serialized.html
+++ b/blocks/test/fixtures/core-embed__instagram.serialized.html
@@ -3,4 +3,4 @@
     https://instagram.com/
     <figcaption>Embedded content from instagram</figcaption>
 </figure>
-<!-- /wp:core-embed/instagram -->
+<!-- /wp:core-embed/instagram -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__issuu.json b/blocks/test/fixtures/core-embed__issuu.json
index 868cd489e5c387..448529e6a8e3a0 100644
--- a/blocks/test/fixtures/core-embed__issuu.json
+++ b/blocks/test/fixtures/core-embed__issuu.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from issuu"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__issuu.serialized.html b/blocks/test/fixtures/core-embed__issuu.serialized.html
index 890348374d3985..af0b652be674d2 100644
--- a/blocks/test/fixtures/core-embed__issuu.serialized.html
+++ b/blocks/test/fixtures/core-embed__issuu.serialized.html
@@ -3,4 +3,4 @@
     https://issuu.com/
     <figcaption>Embedded content from issuu</figcaption>
 </figure>
-<!-- /wp:core-embed/issuu -->
+<!-- /wp:core-embed/issuu -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__kickstarter.json b/blocks/test/fixtures/core-embed__kickstarter.json
index e917cfa1e3385e..4b083df44ac415 100644
--- a/blocks/test/fixtures/core-embed__kickstarter.json
+++ b/blocks/test/fixtures/core-embed__kickstarter.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from kickstarter"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__kickstarter.serialized.html b/blocks/test/fixtures/core-embed__kickstarter.serialized.html
index f3034e6fd96881..cf1057c4a93cd9 100644
--- a/blocks/test/fixtures/core-embed__kickstarter.serialized.html
+++ b/blocks/test/fixtures/core-embed__kickstarter.serialized.html
@@ -3,4 +3,4 @@
     https://kickstarter.com/
     <figcaption>Embedded content from kickstarter</figcaption>
 </figure>
-<!-- /wp:core-embed/kickstarter -->
+<!-- /wp:core-embed/kickstarter -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__meetup-com.json b/blocks/test/fixtures/core-embed__meetup-com.json
index f29968f9082869..9782d21428217c 100644
--- a/blocks/test/fixtures/core-embed__meetup-com.json
+++ b/blocks/test/fixtures/core-embed__meetup-com.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from meetup-com"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__meetup-com.serialized.html b/blocks/test/fixtures/core-embed__meetup-com.serialized.html
index 60334419efc212..85c17eef67e664 100644
--- a/blocks/test/fixtures/core-embed__meetup-com.serialized.html
+++ b/blocks/test/fixtures/core-embed__meetup-com.serialized.html
@@ -3,4 +3,4 @@
     https://meetup.com/
     <figcaption>Embedded content from meetup-com</figcaption>
 </figure>
-<!-- /wp:core-embed/meetup-com -->
+<!-- /wp:core-embed/meetup-com -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__mixcloud.json b/blocks/test/fixtures/core-embed__mixcloud.json
index 1594c158acbe68..75d19946fcd4a8 100644
--- a/blocks/test/fixtures/core-embed__mixcloud.json
+++ b/blocks/test/fixtures/core-embed__mixcloud.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from mixcloud"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__mixcloud.serialized.html b/blocks/test/fixtures/core-embed__mixcloud.serialized.html
index f1945625517031..a31f60256c0ca3 100644
--- a/blocks/test/fixtures/core-embed__mixcloud.serialized.html
+++ b/blocks/test/fixtures/core-embed__mixcloud.serialized.html
@@ -3,4 +3,4 @@
     https://mixcloud.com/
     <figcaption>Embedded content from mixcloud</figcaption>
 </figure>
-<!-- /wp:core-embed/mixcloud -->
+<!-- /wp:core-embed/mixcloud -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__photobucket.json b/blocks/test/fixtures/core-embed__photobucket.json
index f8c2148cbcc77a..45f21b77c80236 100644
--- a/blocks/test/fixtures/core-embed__photobucket.json
+++ b/blocks/test/fixtures/core-embed__photobucket.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from photobucket"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__photobucket.serialized.html b/blocks/test/fixtures/core-embed__photobucket.serialized.html
index af884e8a6be15d..994303b105a489 100644
--- a/blocks/test/fixtures/core-embed__photobucket.serialized.html
+++ b/blocks/test/fixtures/core-embed__photobucket.serialized.html
@@ -3,4 +3,4 @@
     https://photobucket.com/
     <figcaption>Embedded content from photobucket</figcaption>
 </figure>
-<!-- /wp:core-embed/photobucket -->
+<!-- /wp:core-embed/photobucket -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__polldaddy.json b/blocks/test/fixtures/core-embed__polldaddy.json
index 05e4cbeaf045c3..db7ad5fb4ca442 100644
--- a/blocks/test/fixtures/core-embed__polldaddy.json
+++ b/blocks/test/fixtures/core-embed__polldaddy.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from polldaddy"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__polldaddy.serialized.html b/blocks/test/fixtures/core-embed__polldaddy.serialized.html
index 32b560621b43ca..3e46fcad5579da 100644
--- a/blocks/test/fixtures/core-embed__polldaddy.serialized.html
+++ b/blocks/test/fixtures/core-embed__polldaddy.serialized.html
@@ -3,4 +3,4 @@
     https://polldaddy.com/
     <figcaption>Embedded content from polldaddy</figcaption>
 </figure>
-<!-- /wp:core-embed/polldaddy -->
+<!-- /wp:core-embed/polldaddy -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__reddit.json b/blocks/test/fixtures/core-embed__reddit.json
index a04125b76926be..4a6f21f8b975cd 100644
--- a/blocks/test/fixtures/core-embed__reddit.json
+++ b/blocks/test/fixtures/core-embed__reddit.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from reddit"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__reddit.serialized.html b/blocks/test/fixtures/core-embed__reddit.serialized.html
index d498ea438065d9..bc9bc4ca9cc596 100644
--- a/blocks/test/fixtures/core-embed__reddit.serialized.html
+++ b/blocks/test/fixtures/core-embed__reddit.serialized.html
@@ -3,4 +3,4 @@
     https://reddit.com/
     <figcaption>Embedded content from reddit</figcaption>
 </figure>
-<!-- /wp:core-embed/reddit -->
+<!-- /wp:core-embed/reddit -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__reverbnation.json b/blocks/test/fixtures/core-embed__reverbnation.json
index de0f5c62ef564d..404380fdb0703d 100644
--- a/blocks/test/fixtures/core-embed__reverbnation.json
+++ b/blocks/test/fixtures/core-embed__reverbnation.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from reverbnation"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__reverbnation.serialized.html b/blocks/test/fixtures/core-embed__reverbnation.serialized.html
index 3897cf035426b0..482b5406ae7436 100644
--- a/blocks/test/fixtures/core-embed__reverbnation.serialized.html
+++ b/blocks/test/fixtures/core-embed__reverbnation.serialized.html
@@ -3,4 +3,4 @@
     https://reverbnation.com/
     <figcaption>Embedded content from reverbnation</figcaption>
 </figure>
-<!-- /wp:core-embed/reverbnation -->
+<!-- /wp:core-embed/reverbnation -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__screencast.json b/blocks/test/fixtures/core-embed__screencast.json
index a34839a87c2435..77d5edcd8113c1 100644
--- a/blocks/test/fixtures/core-embed__screencast.json
+++ b/blocks/test/fixtures/core-embed__screencast.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from screencast"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__screencast.serialized.html b/blocks/test/fixtures/core-embed__screencast.serialized.html
index 31a004a6d1c1ec..9f583032659b90 100644
--- a/blocks/test/fixtures/core-embed__screencast.serialized.html
+++ b/blocks/test/fixtures/core-embed__screencast.serialized.html
@@ -3,4 +3,4 @@
     https://screencast.com/
     <figcaption>Embedded content from screencast</figcaption>
 </figure>
-<!-- /wp:core-embed/screencast -->
+<!-- /wp:core-embed/screencast -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__scribd.json b/blocks/test/fixtures/core-embed__scribd.json
index 81789fe717be3a..d87c507dc25bf4 100644
--- a/blocks/test/fixtures/core-embed__scribd.json
+++ b/blocks/test/fixtures/core-embed__scribd.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from scribd"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__scribd.serialized.html b/blocks/test/fixtures/core-embed__scribd.serialized.html
index 5adcd8d0a55ae0..7769847c701ffe 100644
--- a/blocks/test/fixtures/core-embed__scribd.serialized.html
+++ b/blocks/test/fixtures/core-embed__scribd.serialized.html
@@ -3,4 +3,4 @@
     https://scribd.com/
     <figcaption>Embedded content from scribd</figcaption>
 </figure>
-<!-- /wp:core-embed/scribd -->
+<!-- /wp:core-embed/scribd -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__slideshare.json b/blocks/test/fixtures/core-embed__slideshare.json
index db0f30df975a9f..da9fa1915a7443 100644
--- a/blocks/test/fixtures/core-embed__slideshare.json
+++ b/blocks/test/fixtures/core-embed__slideshare.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from slideshare"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__slideshare.serialized.html b/blocks/test/fixtures/core-embed__slideshare.serialized.html
index 1149779e78c7e9..3fafcb4dc63425 100644
--- a/blocks/test/fixtures/core-embed__slideshare.serialized.html
+++ b/blocks/test/fixtures/core-embed__slideshare.serialized.html
@@ -3,4 +3,4 @@
     https://slideshare.com/
     <figcaption>Embedded content from slideshare</figcaption>
 </figure>
-<!-- /wp:core-embed/slideshare -->
+<!-- /wp:core-embed/slideshare -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__smugmug.json b/blocks/test/fixtures/core-embed__smugmug.json
index 5b705c05e5c7d5..0dd566a0ea7487 100644
--- a/blocks/test/fixtures/core-embed__smugmug.json
+++ b/blocks/test/fixtures/core-embed__smugmug.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from smugmug"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__smugmug.serialized.html b/blocks/test/fixtures/core-embed__smugmug.serialized.html
index 01abea919b0d1b..fe3cf00c3fff24 100644
--- a/blocks/test/fixtures/core-embed__smugmug.serialized.html
+++ b/blocks/test/fixtures/core-embed__smugmug.serialized.html
@@ -3,4 +3,4 @@
     https://smugmug.com/
     <figcaption>Embedded content from smugmug</figcaption>
 </figure>
-<!-- /wp:core-embed/smugmug -->
+<!-- /wp:core-embed/smugmug -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__soundcloud.json b/blocks/test/fixtures/core-embed__soundcloud.json
index 210360add6681b..dae423da3935cf 100644
--- a/blocks/test/fixtures/core-embed__soundcloud.json
+++ b/blocks/test/fixtures/core-embed__soundcloud.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from soundcloud"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__soundcloud.serialized.html b/blocks/test/fixtures/core-embed__soundcloud.serialized.html
index f5414ef81c99cf..4e11ca9e13f54d 100644
--- a/blocks/test/fixtures/core-embed__soundcloud.serialized.html
+++ b/blocks/test/fixtures/core-embed__soundcloud.serialized.html
@@ -3,4 +3,4 @@
     https://soundcloud.com/
     <figcaption>Embedded content from soundcloud</figcaption>
 </figure>
-<!-- /wp:core-embed/soundcloud -->
+<!-- /wp:core-embed/soundcloud -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__speaker.json b/blocks/test/fixtures/core-embed__speaker.json
index 7fea3ba89edbde..8ec77be522d1bc 100644
--- a/blocks/test/fixtures/core-embed__speaker.json
+++ b/blocks/test/fixtures/core-embed__speaker.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from speaker"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__speaker.serialized.html b/blocks/test/fixtures/core-embed__speaker.serialized.html
index 2e3b4592cbc357..d98ad1a2e744ec 100644
--- a/blocks/test/fixtures/core-embed__speaker.serialized.html
+++ b/blocks/test/fixtures/core-embed__speaker.serialized.html
@@ -3,4 +3,4 @@
     https://speaker.com/
     <figcaption>Embedded content from speaker</figcaption>
 </figure>
-<!-- /wp:core-embed/speaker -->
+<!-- /wp:core-embed/speaker -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__spotify.json b/blocks/test/fixtures/core-embed__spotify.json
index 2e1076dbb0c5fd..431f3c2884b494 100644
--- a/blocks/test/fixtures/core-embed__spotify.json
+++ b/blocks/test/fixtures/core-embed__spotify.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from spotify"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__spotify.serialized.html b/blocks/test/fixtures/core-embed__spotify.serialized.html
index 1fc73903acb3e1..4872c2a4ed9d99 100644
--- a/blocks/test/fixtures/core-embed__spotify.serialized.html
+++ b/blocks/test/fixtures/core-embed__spotify.serialized.html
@@ -3,4 +3,4 @@
     https://spotify.com/
     <figcaption>Embedded content from spotify</figcaption>
 </figure>
-<!-- /wp:core-embed/spotify -->
+<!-- /wp:core-embed/spotify -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__ted.json b/blocks/test/fixtures/core-embed__ted.json
index 5011154882c69f..43d815e12eeea2 100644
--- a/blocks/test/fixtures/core-embed__ted.json
+++ b/blocks/test/fixtures/core-embed__ted.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from ted"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__ted.serialized.html b/blocks/test/fixtures/core-embed__ted.serialized.html
index 01eae201b6b924..164b87706001eb 100644
--- a/blocks/test/fixtures/core-embed__ted.serialized.html
+++ b/blocks/test/fixtures/core-embed__ted.serialized.html
@@ -3,4 +3,4 @@
     https://ted.com/
     <figcaption>Embedded content from ted</figcaption>
 </figure>
-<!-- /wp:core-embed/ted -->
+<!-- /wp:core-embed/ted -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__tumblr.json b/blocks/test/fixtures/core-embed__tumblr.json
index ba37f49055801d..9c799a519180b4 100644
--- a/blocks/test/fixtures/core-embed__tumblr.json
+++ b/blocks/test/fixtures/core-embed__tumblr.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from tumblr"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__tumblr.serialized.html b/blocks/test/fixtures/core-embed__tumblr.serialized.html
index d558986629124a..f69836f3c01147 100644
--- a/blocks/test/fixtures/core-embed__tumblr.serialized.html
+++ b/blocks/test/fixtures/core-embed__tumblr.serialized.html
@@ -3,4 +3,4 @@
     https://tumblr.com/
     <figcaption>Embedded content from tumblr</figcaption>
 </figure>
-<!-- /wp:core-embed/tumblr -->
+<!-- /wp:core-embed/tumblr -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__twitter.json b/blocks/test/fixtures/core-embed__twitter.json
index ff35783fbeeaa9..41aa2a510a114d 100644
--- a/blocks/test/fixtures/core-embed__twitter.json
+++ b/blocks/test/fixtures/core-embed__twitter.json
@@ -7,6 +7,7 @@
             "caption": [
                 "We are Automattic"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__twitter.serialized.html b/blocks/test/fixtures/core-embed__twitter.serialized.html
index 155eaaccbfcd04..537f6e359be2fe 100644
--- a/blocks/test/fixtures/core-embed__twitter.serialized.html
+++ b/blocks/test/fixtures/core-embed__twitter.serialized.html
@@ -3,4 +3,4 @@
     https://twitter.com/automattic
     <figcaption>We are Automattic</figcaption>
 </figure>
-<!-- /wp:core-embed/twitter -->
+<!-- /wp:core-embed/twitter -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__videopress.json b/blocks/test/fixtures/core-embed__videopress.json
index 65e9458e95d877..49f639f7648b7c 100644
--- a/blocks/test/fixtures/core-embed__videopress.json
+++ b/blocks/test/fixtures/core-embed__videopress.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from videopress"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__videopress.serialized.html b/blocks/test/fixtures/core-embed__videopress.serialized.html
index 1b3e5f77597a21..98a77afdee43bf 100644
--- a/blocks/test/fixtures/core-embed__videopress.serialized.html
+++ b/blocks/test/fixtures/core-embed__videopress.serialized.html
@@ -3,4 +3,4 @@
     https://videopress.com/
     <figcaption>Embedded content from videopress</figcaption>
 </figure>
-<!-- /wp:core-embed/videopress -->
+<!-- /wp:core-embed/videopress -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__vimeo.json b/blocks/test/fixtures/core-embed__vimeo.json
index a1f215beed6a8b..c8ad403ddd7e08 100644
--- a/blocks/test/fixtures/core-embed__vimeo.json
+++ b/blocks/test/fixtures/core-embed__vimeo.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from vimeo"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__vimeo.serialized.html b/blocks/test/fixtures/core-embed__vimeo.serialized.html
index 7f1a6d8889aa5d..76df2dfff0c2ed 100644
--- a/blocks/test/fixtures/core-embed__vimeo.serialized.html
+++ b/blocks/test/fixtures/core-embed__vimeo.serialized.html
@@ -3,4 +3,4 @@
     https://vimeo.com/
     <figcaption>Embedded content from vimeo</figcaption>
 </figure>
-<!-- /wp:core-embed/vimeo -->
+<!-- /wp:core-embed/vimeo -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__vine.json b/blocks/test/fixtures/core-embed__vine.json
index af44e8956044ca..2dee557086683a 100644
--- a/blocks/test/fixtures/core-embed__vine.json
+++ b/blocks/test/fixtures/core-embed__vine.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from vine"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__vine.serialized.html b/blocks/test/fixtures/core-embed__vine.serialized.html
index 2681598e22b3fd..504267ab8fbca4 100644
--- a/blocks/test/fixtures/core-embed__vine.serialized.html
+++ b/blocks/test/fixtures/core-embed__vine.serialized.html
@@ -3,4 +3,4 @@
     https://vine.com/
     <figcaption>Embedded content from vine</figcaption>
 </figure>
-<!-- /wp:core-embed/vine -->
+<!-- /wp:core-embed/vine -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__wordpress-tv.json b/blocks/test/fixtures/core-embed__wordpress-tv.json
index 8bf8d711faad06..ffacbfb1e479db 100644
--- a/blocks/test/fixtures/core-embed__wordpress-tv.json
+++ b/blocks/test/fixtures/core-embed__wordpress-tv.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from wordpress-tv"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__wordpress-tv.serialized.html b/blocks/test/fixtures/core-embed__wordpress-tv.serialized.html
index 5fbb3ea107940a..72683e8a4a1a9e 100644
--- a/blocks/test/fixtures/core-embed__wordpress-tv.serialized.html
+++ b/blocks/test/fixtures/core-embed__wordpress-tv.serialized.html
@@ -3,4 +3,4 @@
     https://wordpress.tv/
     <figcaption>Embedded content from wordpress-tv</figcaption>
 </figure>
-<!-- /wp:core-embed/wordpress-tv -->
+<!-- /wp:core-embed/wordpress-tv -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__wordpress.json b/blocks/test/fixtures/core-embed__wordpress.json
index eb24c5f14258d0..041a2167384624 100644
--- a/blocks/test/fixtures/core-embed__wordpress.json
+++ b/blocks/test/fixtures/core-embed__wordpress.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from WordPress"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__wordpress.serialized.html b/blocks/test/fixtures/core-embed__wordpress.serialized.html
index 1543bb764978bd..e6aa8db231881a 100644
--- a/blocks/test/fixtures/core-embed__wordpress.serialized.html
+++ b/blocks/test/fixtures/core-embed__wordpress.serialized.html
@@ -3,4 +3,4 @@
     https://wordpress.com/
     <figcaption>Embedded content from WordPress</figcaption>
 </figure>
-<!-- /wp:core-embed/wordpress -->
+<!-- /wp:core-embed/wordpress -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core-embed__youtube.json b/blocks/test/fixtures/core-embed__youtube.json
index e03983346dcee9..34a7768e4b612b 100644
--- a/blocks/test/fixtures/core-embed__youtube.json
+++ b/blocks/test/fixtures/core-embed__youtube.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from youtube"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core-embed__youtube.serialized.html b/blocks/test/fixtures/core-embed__youtube.serialized.html
index bf575266319a1d..d645549fc87830 100644
--- a/blocks/test/fixtures/core-embed__youtube.serialized.html
+++ b/blocks/test/fixtures/core-embed__youtube.serialized.html
@@ -3,4 +3,4 @@
     https://youtube.com/
     <figcaption>Embedded content from youtube</figcaption>
 </figure>
-<!-- /wp:core-embed/youtube -->
+<!-- /wp:core-embed/youtube -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__button__center.json b/blocks/test/fixtures/core__button__center.json
index 6a257c42322e77..ab6d2a2e4babc4 100644
--- a/blocks/test/fixtures/core__button__center.json
+++ b/blocks/test/fixtures/core__button__center.json
@@ -8,6 +8,7 @@
             "text": [
                 "Help build Gutenberg"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__button__center.serialized.html b/blocks/test/fixtures/core__button__center.serialized.html
index 0376ad37ecc3cb..8e085b558c48f6 100644
--- a/blocks/test/fixtures/core__button__center.serialized.html
+++ b/blocks/test/fixtures/core__button__center.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/button {"align":"center"} -->
 <div class="wp-block-button aligncenter"><a href="https://github.com/WordPress/gutenberg">Help build Gutenberg</a></div>
-<!-- /wp:core/button -->
+<!-- /wp:core/button -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__code.json b/blocks/test/fixtures/core__code.json
index 4d2fc33c743b73..9a0d8b4ec10a3c 100644
--- a/blocks/test/fixtures/core__code.json
+++ b/blocks/test/fixtures/core__code.json
@@ -4,6 +4,7 @@
         "name": "core/code",
         "attributes": {
             "content": "export default function MyButton() {\n\treturn <Button>Click Me!</Button>;\n}"
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__code.serialized.html b/blocks/test/fixtures/core__code.serialized.html
index 5d3f951c17f71c..ba8ea3bed4cbdd 100644
--- a/blocks/test/fixtures/core__code.serialized.html
+++ b/blocks/test/fixtures/core__code.serialized.html
@@ -2,4 +2,4 @@
 <pre class="wp-block-code"><code>export default function MyButton() {
 	return &lt;Button&gt;Click Me!&lt;/Button&gt;;
 }</code></pre>
-<!-- /wp:core/code -->
+<!-- /wp:core/code -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__cover-image.json b/blocks/test/fixtures/core__cover-image.json
index e76615bc0388e9..513aedf310c6b7 100644
--- a/blocks/test/fixtures/core__cover-image.json
+++ b/blocks/test/fixtures/core__cover-image.json
@@ -5,6 +5,7 @@
         "attributes": {
             "url": "https://cldup.com/uuUqE_dXzy.jpg",
             "title": "Guten Berg!"
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__cover-image.serialized.html b/blocks/test/fixtures/core__cover-image.serialized.html
index 71dbaaf017f04c..e56ce920fbf162 100644
--- a/blocks/test/fixtures/core__cover-image.serialized.html
+++ b/blocks/test/fixtures/core__cover-image.serialized.html
@@ -4,4 +4,4 @@
         <h2>Guten Berg!</h2>
     </section>
 </section>
-<!-- /wp:core/cover-image -->
+<!-- /wp:core/cover-image -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__embed.json b/blocks/test/fixtures/core__embed.json
index f9f5ee6c346cda..0182b54a730fe4 100644
--- a/blocks/test/fixtures/core__embed.json
+++ b/blocks/test/fixtures/core__embed.json
@@ -7,6 +7,7 @@
             "caption": [
                 "Embedded content from an example URL"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__embed.serialized.html b/blocks/test/fixtures/core__embed.serialized.html
index a75136b0413ef7..6ec5ef182bce1c 100644
--- a/blocks/test/fixtures/core__embed.serialized.html
+++ b/blocks/test/fixtures/core__embed.serialized.html
@@ -3,4 +3,4 @@
     https://example.com/
     <figcaption>Embedded content from an example URL</figcaption>
 </figure>
-<!-- /wp:core/embed -->
+<!-- /wp:core/embed -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__freeform.json b/blocks/test/fixtures/core__freeform.json
index 35efd7b04fd19f..da8f65857a79df 100644
--- a/blocks/test/fixtures/core__freeform.json
+++ b/blocks/test/fixtures/core__freeform.json
@@ -4,6 +4,7 @@
         "name": "core/freeform",
         "attributes": {
             "content": "Testing freeform block with some\n<div class=\"wp-some-class\">\n\tHTML <span style=\"color: red;\">content</span>\n</div>"
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__freeform.serialized.html b/blocks/test/fixtures/core__freeform.serialized.html
index 0bf5ad6d0d8292..232ec49ed7ff1c 100644
--- a/blocks/test/fixtures/core__freeform.serialized.html
+++ b/blocks/test/fixtures/core__freeform.serialized.html
@@ -3,4 +3,4 @@
 <div class="wp-some-class">
     HTML <span style="color: red;">content</span>
 </div>
-<!-- /wp:core/freeform -->
+<!-- /wp:core/freeform -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__gallery.json b/blocks/test/fixtures/core__gallery.json
index aa42f70c3990a2..b31406181c5191 100644
--- a/blocks/test/fixtures/core__gallery.json
+++ b/blocks/test/fixtures/core__gallery.json
@@ -37,6 +37,8 @@
                     "alt": "title"
                 }
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<div class=\"wp-block-gallery\">\n\t<figure class=\"blocks-gallery-image\">\n\t\t<img src=\"https://cldup.com/uuUqE_dXzy.jpg\" alt=\"title\" />\n\t</figure>\n\t<figure class=\"blocks-gallery-image\">\n\t\t<img src=\"http://google.com/hi.png\" alt=\"title\" />\n\t</figure>\n</div>"
     }
 ]
diff --git a/blocks/test/fixtures/core__gallery.serialized.html b/blocks/test/fixtures/core__gallery.serialized.html
index f3be93a9cc842e..1d031e16635d3c 100644
--- a/blocks/test/fixtures/core__gallery.serialized.html
+++ b/blocks/test/fixtures/core__gallery.serialized.html
@@ -1,6 +1,10 @@
 <!-- wp:core/gallery {"images":[{"sizes":{"full":{"url":"https://cldup.com/uuUqE_dXzy.jpg","height":1080,"width":810,"orientation":"portrait"}},"mime":"image/jpeg","type":"image","subtype":"jpeg","id":1,"url":"https://cldup.com/uuUqE_dXzy.jpg","alt":"title"},{"sizes":{"full":{"url":"http://google.com/hi.png","height":1080,"width":1440,"orientation":"landscape"}},"mime":"image/jpeg","type":"image","subtype":"jpeg","id":2,"url":"http://google.com/hi.png","alt":"title"}]} -->
-<div class="wp-block-gallery alignnone columns-2 is-cropped">
-    <figure class="blocks-gallery-image"><img src="https://cldup.com/uuUqE_dXzy.jpg" alt="title" /></figure>
-    <figure class="blocks-gallery-image"><img src="http://google.com/hi.png" alt="title" /></figure>
+<div class="wp-block-gallery">
+    <figure class="blocks-gallery-image">
+        <img src="https://cldup.com/uuUqE_dXzy.jpg" alt="title" />
+    </figure>
+    <figure class="blocks-gallery-image">
+        <img src="http://google.com/hi.png" alt="title" />
+    </figure>
 </div>
-<!-- /wp:core/gallery -->
+<!-- /wp:core/gallery -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__heading__h2-em.json b/blocks/test/fixtures/core__heading__h2-em.json
index 16513a34cfa3cc..9ca792adf98b9e 100644
--- a/blocks/test/fixtures/core__heading__h2-em.json
+++ b/blocks/test/fixtures/core__heading__h2-em.json
@@ -3,6 +3,7 @@
         "uid": "_uid_0",
         "name": "core/heading",
         "attributes": {
+            "nodeName": "H2",
             "content": [
                 "The ",
                 {
@@ -10,8 +11,8 @@
                     "children": "Inserter"
                 },
                 " Tool"
-            ],
-            "nodeName": "H2"
-        }
+            ]
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__heading__h2-em.serialized.html b/blocks/test/fixtures/core__heading__h2-em.serialized.html
index 7755d1dcf4eae2..6ca13a28d644c5 100644
--- a/blocks/test/fixtures/core__heading__h2-em.serialized.html
+++ b/blocks/test/fixtures/core__heading__h2-em.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/heading -->
 <h2>The <em>Inserter</em> Tool</h2>
-<!-- /wp:core/heading -->
+<!-- /wp:core/heading -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__heading__h2.json b/blocks/test/fixtures/core__heading__h2.json
index 915407fa11b90d..974fc08c5d3277 100644
--- a/blocks/test/fixtures/core__heading__h2.json
+++ b/blocks/test/fixtures/core__heading__h2.json
@@ -3,10 +3,11 @@
         "uid": "_uid_0",
         "name": "core/heading",
         "attributes": {
+            "nodeName": "H2",
             "content": [
                 "A picture is worth a thousand words, or so the saying goes"
-            ],
-            "nodeName": "H2"
-        }
+            ]
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__heading__h2.serialized.html b/blocks/test/fixtures/core__heading__h2.serialized.html
index a55b391a250578..d61ccfe498c4cf 100644
--- a/blocks/test/fixtures/core__heading__h2.serialized.html
+++ b/blocks/test/fixtures/core__heading__h2.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/heading -->
 <h2>A picture is worth a thousand words, or so the saying goes</h2>
-<!-- /wp:core/heading -->
+<!-- /wp:core/heading -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__html.json b/blocks/test/fixtures/core__html.json
index c5f2daf6b551bc..75f0de81f30286 100644
--- a/blocks/test/fixtures/core__html.json
+++ b/blocks/test/fixtures/core__html.json
@@ -4,6 +4,7 @@
         "name": "core/html",
         "attributes": {
             "content": "<h1>Some HTML code</h1>\n<marquee>This text will scroll from right to left</marquee>"
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__html.serialized.html b/blocks/test/fixtures/core__html.serialized.html
index 4824e6e373c06e..b101b290afabe1 100644
--- a/blocks/test/fixtures/core__html.serialized.html
+++ b/blocks/test/fixtures/core__html.serialized.html
@@ -1,4 +1,4 @@
 <!-- wp:core/html -->
 <h1>Some HTML code</h1>
 <marquee>This text will scroll from right to left</marquee>
-<!-- /wp:core/html -->
+<!-- /wp:core/html -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__image.json b/blocks/test/fixtures/core__image.json
index e04ca6120b14a6..fda4029f28ba9d 100644
--- a/blocks/test/fixtures/core__image.json
+++ b/blocks/test/fixtures/core__image.json
@@ -5,6 +5,7 @@
         "attributes": {
             "url": "https://cldup.com/uuUqE_dXzy.jpg",
             "caption": []
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__image.serialized.html b/blocks/test/fixtures/core__image.serialized.html
index 2a414855b03441..d71f8efb0b17dc 100644
--- a/blocks/test/fixtures/core__image.serialized.html
+++ b/blocks/test/fixtures/core__image.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/image -->
 <figure class="wp-block-image"><img src="https://cldup.com/uuUqE_dXzy.jpg" /></figure>
-<!-- /wp:core/image -->
+<!-- /wp:core/image -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__image__center-caption.json b/blocks/test/fixtures/core__image__center-caption.json
index 4630b4bb896828..e8190233e4a655 100644
--- a/blocks/test/fixtures/core__image__center-caption.json
+++ b/blocks/test/fixtures/core__image__center-caption.json
@@ -8,6 +8,8 @@
             "caption": [
                 "Give it a try. Press the \"really wide\" button on the image toolbar."
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<figure class=\"wp-block-image\"><img src=\"https://cldup.com/YLYhpou2oq.jpg\" class=\"aligncenter\"/><figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption></figure>"
     }
 ]
diff --git a/blocks/test/fixtures/core__image__center-caption.serialized.html b/blocks/test/fixtures/core__image__center-caption.serialized.html
index 75b4be62d86b87..c0e7bcca2139aa 100644
--- a/blocks/test/fixtures/core__image__center-caption.serialized.html
+++ b/blocks/test/fixtures/core__image__center-caption.serialized.html
@@ -1,5 +1,5 @@
 <!-- wp:core/image {"align":"center"} -->
-<figure class="wp-block-image aligncenter"><img src="https://cldup.com/YLYhpou2oq.jpg" />
+<figure class="wp-block-image"><img src="https://cldup.com/YLYhpou2oq.jpg" class="aligncenter" />
     <figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption>
 </figure>
-<!-- /wp:core/image -->
+<!-- /wp:core/image -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__latest-posts.json b/blocks/test/fixtures/core__latest-posts.json
index f1ee6bdaf09f2d..28300d2f0940d2 100644
--- a/blocks/test/fixtures/core__latest-posts.json
+++ b/blocks/test/fixtures/core__latest-posts.json
@@ -4,8 +4,9 @@
         "name": "core/latest-posts",
         "attributes": {
             "postsToShow": 5,
-            "layout": "list",
-            "displayPostDate": false
-        }
+            "displayPostDate": false,
+            "layout": "list"
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__latest-posts.serialized.html b/blocks/test/fixtures/core__latest-posts.serialized.html
index 92aef9efca10f1..e9b8232ccd242b 100644
--- a/blocks/test/fixtures/core__latest-posts.serialized.html
+++ b/blocks/test/fixtures/core__latest-posts.serialized.html
@@ -1 +1 @@
-<!-- wp:core/latest-posts {"postsToShow":5,"displayPostDate":false,"layout":"list"} /-->
+<!-- wp:core/latest-posts {"postsToShow":5,"displayPostDate":false,"layout":"list"} /-->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__list__ul.json b/blocks/test/fixtures/core__list__ul.json
index 1ebd8c3f7fd287..005f648e24193f 100644
--- a/blocks/test/fixtures/core__list__ul.json
+++ b/blocks/test/fixtures/core__list__ul.json
@@ -37,6 +37,8 @@
                     ]
                 }
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<ul><li>Text & Headings</li><li>Images & Videos</li><li>Galleries</li><li>Embeds, like YouTube, Tweets, or other WordPress posts.</li><li>Layout blocks, like Buttons, Hero Images, Separators, etc.</li><li>And <em>Lists</em> like this one of course :)</li></ul>"
     }
 ]
diff --git a/blocks/test/fixtures/core__list__ul.serialized.html b/blocks/test/fixtures/core__list__ul.serialized.html
index d5636ea9b38131..61fb3b02a70440 100644
--- a/blocks/test/fixtures/core__list__ul.serialized.html
+++ b/blocks/test/fixtures/core__list__ul.serialized.html
@@ -1,10 +1,10 @@
 <!-- wp:core/list -->
 <ul>
-    <li>Text &amp; Headings</li>
-    <li>Images &amp; Videos</li>
+    <li>Text & Headings</li>
+    <li>Images & Videos</li>
     <li>Galleries</li>
     <li>Embeds, like YouTube, Tweets, or other WordPress posts.</li>
     <li>Layout blocks, like Buttons, Hero Images, Separators, etc.</li>
     <li>And <em>Lists</em> like this one of course :)</li>
 </ul>
-<!-- /wp:core/list -->
+<!-- /wp:core/list -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__preformatted.json b/blocks/test/fixtures/core__preformatted.json
index 3b77060722d570..77de62f03bda6e 100644
--- a/blocks/test/fixtures/core__preformatted.json
+++ b/blocks/test/fixtures/core__preformatted.json
@@ -15,6 +15,8 @@
                 },
                 "And more!"
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<pre class=\"wp-block-preformatted\">Some <em>preformatted</em> text...<br>And more!</pre>"
     }
 ]
diff --git a/blocks/test/fixtures/core__preformatted.serialized.html b/blocks/test/fixtures/core__preformatted.serialized.html
index b8bceecf41f82e..0064f0f01cf017 100644
--- a/blocks/test/fixtures/core__preformatted.serialized.html
+++ b/blocks/test/fixtures/core__preformatted.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/preformatted -->
-<pre class="wp-block-preformatted">Some <em>preformatted</em> text...<br/>And more!</pre>
-<!-- /wp:core/preformatted -->
+<pre class="wp-block-preformatted">Some <em>preformatted</em> text...<br>And more!</pre>
+<!-- /wp:core/preformatted -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__pullquote.json b/blocks/test/fixtures/core__pullquote.json
index 088e26a2d92f0a..53e5e64c35c05c 100644
--- a/blocks/test/fixtures/core__pullquote.json
+++ b/blocks/test/fixtures/core__pullquote.json
@@ -11,6 +11,8 @@
             "citation": [
                 "...with a caption"
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<blockquote class=\"wp-block-pullquote\">\n<p>Testing pullquote block...</p><footer>...with a caption</footer>\n</blockquote>"
     }
 ]
diff --git a/blocks/test/fixtures/core__pullquote.serialized.html b/blocks/test/fixtures/core__pullquote.serialized.html
index 2957236d56242b..be431d695e3be8 100644
--- a/blocks/test/fixtures/core__pullquote.serialized.html
+++ b/blocks/test/fixtures/core__pullquote.serialized.html
@@ -1,6 +1,6 @@
 <!-- wp:core/pullquote -->
-<blockquote class="wp-block-pullquote alignnone">
+<blockquote class="wp-block-pullquote">
     <p>Testing pullquote block...</p>
     <footer>...with a caption</footer>
 </blockquote>
-<!-- /wp:core/pullquote -->
+<!-- /wp:core/pullquote -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__quote__style-1.json b/blocks/test/fixtures/core__quote__style-1.json
index dd76ad46a3da74..f317088d34d1a4 100644
--- a/blocks/test/fixtures/core__quote__style-1.json
+++ b/blocks/test/fixtures/core__quote__style-1.json
@@ -3,16 +3,17 @@
         "uid": "_uid_0",
         "name": "core/quote",
         "attributes": {
-            "style": "1",
             "value": [
                 {
-                    "children": "The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.",
-                    "type": "p"
+                    "type": "p",
+                    "children": "The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery."
                 }
             ],
+            "style": "1",
             "citation": [
                 "Matt Mullenweg, 2017"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__quote__style-1.serialized.html b/blocks/test/fixtures/core__quote__style-1.serialized.html
index b6f8ab20095fc7..6f6d6c0ae15077 100644
--- a/blocks/test/fixtures/core__quote__style-1.serialized.html
+++ b/blocks/test/fixtures/core__quote__style-1.serialized.html
@@ -3,4 +3,4 @@
     <p>The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.</p>
     <footer>Matt Mullenweg, 2017</footer>
 </blockquote>
-<!-- /wp:core/quote -->
+<!-- /wp:core/quote -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__quote__style-2.json b/blocks/test/fixtures/core__quote__style-2.json
index 114a32462aea3a..89950d45b7167a 100644
--- a/blocks/test/fixtures/core__quote__style-2.json
+++ b/blocks/test/fixtures/core__quote__style-2.json
@@ -3,16 +3,17 @@
         "uid": "_uid_0",
         "name": "core/quote",
         "attributes": {
-            "style": "2",
             "value": [
                 {
-                    "children": "There is no greater agony than bearing an untold story inside you.",
-                    "type": "p"
+                    "type": "p",
+                    "children": "There is no greater agony than bearing an untold story inside you."
                 }
             ],
+            "style": "2",
             "citation": [
                 "Maya Angelou"
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__quote__style-2.serialized.html b/blocks/test/fixtures/core__quote__style-2.serialized.html
index 109f466bbd19c5..8b567a80e83715 100644
--- a/blocks/test/fixtures/core__quote__style-2.serialized.html
+++ b/blocks/test/fixtures/core__quote__style-2.serialized.html
@@ -3,4 +3,4 @@
     <p>There is no greater agony than bearing an untold story inside you.</p>
     <footer>Maya Angelou</footer>
 </blockquote>
-<!-- /wp:core/quote -->
+<!-- /wp:core/quote -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__separator.json b/blocks/test/fixtures/core__separator.json
index c068dfcf2de5a3..53bdd684bcb6fe 100644
--- a/blocks/test/fixtures/core__separator.json
+++ b/blocks/test/fixtures/core__separator.json
@@ -2,6 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/separator",
-        "attributes": {}
+        "attributes": {},
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__separator.serialized.html b/blocks/test/fixtures/core__separator.serialized.html
index d835a483147f13..5e3a7c0714b35d 100644
--- a/blocks/test/fixtures/core__separator.serialized.html
+++ b/blocks/test/fixtures/core__separator.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/separator -->
 <hr class="wp-block-separator" />
-<!-- /wp:core/separator -->
+<!-- /wp:core/separator -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__table.json b/blocks/test/fixtures/core__table.json
index 46d03ed4a7a909..c0cd91a19bc691 100644
--- a/blocks/test/fixtures/core__table.json
+++ b/blocks/test/fixtures/core__table.json
@@ -194,6 +194,8 @@
                     ]
                 }
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<table><thead><tr><th>Version</th><th>Musician</th><th>Date</th></tr></thead><tbody><tr><td><a href=\"https://wordpress.org/news/2003/05/wordpress-now-available/\">.70</a></td><td>No musician chosen.</td><td>May 27, 2003</td></tr><tr><td><a href=\"https://wordpress.org/news/2004/01/wordpress-10/\">1.0</a></td><td>Miles Davis</td><td>January 3, 2004</td></tr><tr><td>Lots of versions skipped, see <a href=\"https://codex.wordpress.org/WordPress_Versions\">the full list</a></td><td>&hellip;</td><td>&hellip;</td></tr><tr><td><a href=\"https://wordpress.org/news/2015/12/clifford/\">4.4</a></td><td>Clifford Brown</td><td>December 8, 2015</td></tr><tr><td><a href=\"https://wordpress.org/news/2016/04/coleman/\">4.5</a></td><td>Coleman Hawkins</td><td>April 12, 2016</td></tr><tr><td><a href=\"https://wordpress.org/news/2016/08/pepper/\">4.6</a></td><td>Pepper Adams</td><td>August 16, 2016</td></tr><tr><td><a href=\"https://wordpress.org/news/2016/12/vaughan/\">4.7</a></td><td>Sarah Vaughan</td><td>December 6, 2016</td></tr></tbody></table>"
     }
 ]
diff --git a/blocks/test/fixtures/core__table.serialized.html b/blocks/test/fixtures/core__table.serialized.html
index 67310f7464fd27..33ecf7901d9522 100644
--- a/blocks/test/fixtures/core__table.serialized.html
+++ b/blocks/test/fixtures/core__table.serialized.html
@@ -1,5 +1,5 @@
 <!-- wp:core/table -->
-<table class="wp-block-table">
+<table>
     <thead>
         <tr>
             <th>Version</th>
@@ -20,8 +20,8 @@
         </tr>
         <tr>
             <td>Lots of versions skipped, see <a href="https://codex.wordpress.org/WordPress_Versions">the full list</a></td>
-            <td>…</td>
-            <td>…</td>
+            <td>&hellip;</td>
+            <td>&hellip;</td>
         </tr>
         <tr>
             <td><a href="https://wordpress.org/news/2015/12/clifford/">4.4</a></td>
diff --git a/blocks/test/fixtures/core__text__align-right.json b/blocks/test/fixtures/core__text__align-right.json
index e41e915f38069c..49b1d5e887f055 100644
--- a/blocks/test/fixtures/core__text__align-right.json
+++ b/blocks/test/fixtures/core__text__align-right.json
@@ -5,8 +5,11 @@
         "attributes": {
             "align": "right",
             "content": [
-                [ "... like this one, which is separate from the above and right aligned." ]
+                [
+                    "... like this one, which is separate from the above and right aligned."
+                ]
             ]
-        }
+        },
+        "isValid": true
     }
 ]
diff --git a/blocks/test/fixtures/core__text__align-right.serialized.html b/blocks/test/fixtures/core__text__align-right.serialized.html
index 65a819fd91c6f2..e841a0450bb7c7 100644
--- a/blocks/test/fixtures/core__text__align-right.serialized.html
+++ b/blocks/test/fixtures/core__text__align-right.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/text {"align":"right"} -->
 <p style="text-align:right;">... like this one, which is separate from the above and right aligned.</p>
-<!-- /wp:core/text -->
+<!-- /wp:core/text -->
\ No newline at end of file
diff --git a/blocks/test/fixtures/core__verse.json b/blocks/test/fixtures/core__verse.json
index 119f0049396dfa..eeb459c0eb5f11 100644
--- a/blocks/test/fixtures/core__verse.json
+++ b/blocks/test/fixtures/core__verse.json
@@ -15,6 +15,8 @@
                 },
                 "And more!"
             ]
-        }
+        },
+        "isValid": false,
+        "originalContent": "<pre class=\"wp-block-verse\">A <em>verse</em>…<br>And more!</pre>"
     }
 ]
diff --git a/blocks/test/fixtures/core__verse.serialized.html b/blocks/test/fixtures/core__verse.serialized.html
index b65e2819131c21..4ecbf337798d1c 100644
--- a/blocks/test/fixtures/core__verse.serialized.html
+++ b/blocks/test/fixtures/core__verse.serialized.html
@@ -1,3 +1,3 @@
 <!-- wp:core/verse -->
-<pre class="wp-block-verse">A <em>verse</em>…<br/>And more!</pre>
-<!-- /wp:core/verse -->
+<pre class="wp-block-verse">A <em>verse</em>…<br>And more!</pre>
+<!-- /wp:core/verse -->
\ No newline at end of file
diff --git a/editor/assets/stylesheets/_z-index.scss b/editor/assets/stylesheets/_z-index.scss
index 8b119e44718786..4fa4e287f04206 100644
--- a/editor/assets/stylesheets/_z-index.scss
+++ b/editor/assets/stylesheets/_z-index.scss
@@ -7,6 +7,7 @@ $z-layers: (
 	'.editor-visual-editor__block:before': -1,
 	'.editor-visual-editor__block {core/image aligned left or right}': 10,
 	'.editor-visual-editor__block-controls': 1,
+	'.editor-visual-editor__invalid-block-warning': 1,
 	'.components-form-toggle__input': 1,
 	'.editor-format-list__menu': 1,
 	'.editor-inserter__tabs': 1,
diff --git a/editor/modes/visual-editor/block.js b/editor/modes/visual-editor/block.js
index fe31c3e54d042e..cf2030cf2f2641 100644
--- a/editor/modes/visual-editor/block.js
+++ b/editor/modes/visual-editor/block.js
@@ -19,6 +19,7 @@ import { __, sprintf } from 'i18n';
 /**
  * Internal dependencies
  */
+import InvalidBlockWarning from './invalid-block-warning';
 import BlockMover from '../../block-mover';
 import BlockRightMenu from '../../block-settings-menu';
 import BlockSwitcher from '../../block-switcher';
@@ -302,10 +303,10 @@ class VisualEditorBlock extends Component {
 
 	render() {
 		const { block, multiSelectedBlockUids } = this.props;
-		const blockType = getBlockType( block.name );
+		const { name: blockName, isValid } = block;
+		const blockType = getBlockType( blockName );
 		// translators: %s: Type of block (i.e. Text, Image etc)
 		const blockLabel = sprintf( __( 'Block: %s' ), blockType.title );
-		const { className = getBlockDefaultClassname( block.name ) } = blockType;
 		// The block as rendered in the editor is composed of general block UI
 		// (mover, toolbar, wrapper) and the display of the block content, which
 		// is referred to as <BlockEdit />.
@@ -327,6 +328,7 @@ class VisualEditorBlock extends Component {
 		const showUI = isSelected && ( ! this.props.isTyping || focus.collapsed === false );
 		const { showMobileControls } = this.state;
 		const wrapperClassname = classnames( 'editor-visual-editor__block', {
+			'is-invalid': ! isValid,
 			'is-selected': showUI,
 			'is-multi-selected': isMultiSelected,
 			'is-hovered': isHovered,
@@ -341,6 +343,10 @@ class VisualEditorBlock extends Component {
 			wrapperProps = blockType.getEditWrapperProps( block.attributes );
 		}
 
+		// Generate a class name for the block's editable form
+		let { className = getBlockDefaultClassname( block.name ) } = blockType;
+		className = classnames( className, block.attributes.className );
+
 		// Disable reason: Each block can be selected by clicking on it
 		/* eslint-disable jsx-a11y/no-static-element-interactions, jsx-a11y/onclick-has-role, jsx-a11y/click-events-have-key-events */
 		return (
@@ -360,7 +366,7 @@ class VisualEditorBlock extends Component {
 			>
 				{ ( showUI || isHovered ) && <BlockMover uids={ [ block.uid ] } /> }
 				{ ( showUI || isHovered ) && <BlockRightMenu uid={ block.uid } /> }
-				{ showUI &&
+				{ showUI && isValid &&
 					<CSSTransitionGroup
 						transitionName={ { appear: 'is-appearing', appearActive: 'is-appearing-active' } }
 						transitionAppear={ true }
@@ -394,18 +400,28 @@ class VisualEditorBlock extends Component {
 					onDragStart={ ( event ) => event.preventDefault() }
 					onMouseDown={ this.onPointerDown }
 					onTouchStart={ this.onPointerDown }
+					className="editor-visual-editor__block-edit"
 				>
-					<BlockEdit
-						focus={ focus }
-						attributes={ block.attributes }
-						setAttributes={ this.setAttributes }
-						insertBlocksAfter={ onInsertBlocksAfter }
-						setFocus={ partial( onFocus, block.uid ) }
-						mergeBlocks={ this.mergeBlocks }
-						className={ classnames( className, block.attributes.className ) }
-						id={ block.uid }
-					/>
+					{ isValid && (
+						<BlockEdit
+							focus={ focus }
+							attributes={ block.attributes }
+							setAttributes={ this.setAttributes }
+							insertBlocksAfter={ onInsertBlocksAfter }
+							setFocus={ partial( onFocus, block.uid ) }
+							mergeBlocks={ this.mergeBlocks }
+							className={ className }
+							id={ block.uid }
+						/>
+					) }
+					{ ! isValid && (
+						blockType.save( {
+							attributes: block.attributes,
+							className,
+						} )
+					) }
 				</div>
+				{ ! isValid && <InvalidBlockWarning /> }
 			</div>
 		);
 		/* eslint-enable jsx-a11y/no-static-element-interactions, jsx-a11y/onclick-has-role, jsx-a11y/click-events-have-key-events */
diff --git a/editor/modes/visual-editor/invalid-block-warning.js b/editor/modes/visual-editor/invalid-block-warning.js
new file mode 100644
index 00000000000000..ad28723c8be2e2
--- /dev/null
+++ b/editor/modes/visual-editor/invalid-block-warning.js
@@ -0,0 +1,17 @@
+/**
+ * WordPress dependencies
+ */
+import { Dashicon } from 'components';
+import { __ } from 'i18n';
+
+const warning = (
+	<div className="editor-visual-editor__invalid-block-warning">
+		<Dashicon icon="warning" />
+		<p>{ __(
+			'This block has been modified externally and has been locked to ' +
+			'protect against content loss.'
+		) }</p>
+	</div>
+);
+
+export default () => warning;
diff --git a/editor/modes/visual-editor/style.scss b/editor/modes/visual-editor/style.scss
index 60d98ad56f790a..39b87a261b03d3 100644
--- a/editor/modes/visual-editor/style.scss
+++ b/editor/modes/visual-editor/style.scss
@@ -56,6 +56,23 @@
 		}
 	}
 
+	&.is-invalid .editor-visual-editor__block-edit {
+		position: relative;
+		min-height: 250px;
+		pointer-events: none;
+		user-select: none;
+	}
+
+	&.is-invalid .editor-visual-editor__block-edit::after {
+		content: '';
+		position: absolute;
+		top: 0;
+		right: 0;
+		bottom: 0;
+		left: 0;
+		background-color: rgba( $white, 0.6 );
+	}
+
 	&.is-hovered:before {
 		outline: 1px solid $light-gray-500;
 		transition: 0.2s outline;
@@ -389,3 +406,28 @@ $sticky-bottom-offset: 20px;
 		display: none;
 	}
 }
+
+.editor-visual-editor__invalid-block-warning {
+	z-index: z-index( '.editor-visual-editor__invalid-block-warning' );
+	position: absolute;
+	top: 50%;
+	left: 50%;
+	transform: translate( -50%, -50% );
+	display: flex;
+	flex-direction: column;
+	justify-content: space-around;
+	align-items: center;
+	width: 96%;
+	max-width: 360px;
+	padding: 20px 20px 10px 20px;
+	background-color: $white;
+	border: 1px solid $light-gray-500;
+	text-align: center;
+	line-height: $default-line-height;
+	box-shadow: $shadow-popover;
+
+	p {
+		font-family: $default-font;
+		font-size: $default-font-size;
+	}
+}
diff --git a/post-content.js b/post-content.js
index 0ea57e8bb22801..3ae070a6fc4222 100644
--- a/post-content.js
+++ b/post-content.js
@@ -7,23 +7,23 @@ window._wpGutenbergPost.title = {
 
 window._wpGutenbergPost.content = {
 	raw: [
-		'<!-- wp:core/cover-image { "url": "https://cldup.com/GCwahb3aOb.jpg", "align": "full", "hasParallax": true } -->',
-		'<section className="cover-image wp-block-cover-image" style={ { backgroundImage: \'url("https://cldup.com/GCwahb3aOb.jpg");\' } }><h2>Of mountains & printing presses</h2></section>',
+		'<!-- wp:core/cover-image {"url":"https://cldup.com/GCwahb3aOb.jpg","align":"full","hasParallax":true} -->',
+		'<section class="wp-block-cover-image cover-image" style="background-image: url(https://cldup.com/GCwahb3aOb.jpg);"><h2>Of mountains &amp; printing presses</h2></section>',
 		'<!-- /wp:core/cover-image -->',
 
 		'<!-- more Keep on reading! -->',
 		'<!-- noteaser -->',
 
-		'<!-- wp:core/text { "project": { "name": "gutenberg", "for": "WordPress" }, "isAwesome": true } -->',
-		'<p>The goal of this new editor is to make adding rich content to WordPress simple and enjoyable. This whole post is composed of <em>pieces of content</em>—somewhat similar to LEGO bricks—that you can move around and interact with. Move your cursor around and you\'ll notice the different blocks light up with outlines and arrows. Press the arrows to reposition blocks quickly, without fearing about losing things in the process of copying and pasting.</p>',
+		'<!-- wp:core/text -->',
+		'<p>The goal of this new editor is to make adding rich content to WordPress simple and enjoyable. This whole post is composed of <em>pieces of content</em>—somewhat similar to LEGO bricks—that you can move around and interact with. Move your cursor around and you&#x27;ll notice the different blocks light up with outlines and arrows. Press the arrows to reposition blocks quickly, without fearing about losing things in the process of copying and pasting.</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/text -->',
 		'<p>What you are reading now is a <strong>text block</strong>, the most basic block of all. The text block has its own controls to be moved freely around the post...</p>',
 		'<!-- /wp:core/text -->',
 
-		'<!-- wp:core/text { "align": "right" } -->',
-		'<p style="text-align:right;">... like this one, which is right aligned.</p>',
+		'<!-- wp:core/text {"align":"right"} -->',
+		'<p style="text-align:right">... like this one, which is right aligned.</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/more /-->',
@@ -37,15 +37,15 @@ window._wpGutenbergPost.content = {
 		'<!-- /wp:core/heading -->',
 
 		'<!-- wp:core/text -->',
-		'<p>Handling images and media with the utmost care is a primary focus of the new editor. Hopefully, you\'ll find aspects of adding captions or going full-width with your pictures much easier and robust than before.</p>',
+		'<p>Handling images and media with the utmost care is a primary focus of the new editor. Hopefully, you&#x27;ll find aspects of adding captions or going full-width with your pictures much easier and robust than before.</p>',
 		'<!-- /wp:core/text -->',
 
-		'<!-- wp:core/image { "align": "center" } -->',
-		'<figure class="wp-block-image"><img alt="Beautiful landscape" src="https://cldup.com/YLYhpou2oq.jpg" class="aligncenter"/><figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption></figure>',
+		'<!-- wp:core/image {"align":"center"} -->',
+		'<figure class="wp-block-image aligncenter"><img src="https://cldup.com/YLYhpou2oq.jpg" alt="Beautiful landscape" /><figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption></figure>',
 		'<!-- /wp:core/image -->',
 
 		'<!-- wp:core/text -->',
-		'<p>Try selecting and removing or editing the caption, now you don\'t have to be careful about selecting the image or other text by mistake and ruining the presentation.</p>',
+		'<p>Try selecting and removing or editing the caption, now you don&#x27;t have to be careful about selecting the image or other text by mistake and ruining the presentation.</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/heading -->',
@@ -53,15 +53,15 @@ window._wpGutenbergPost.content = {
 		'<!-- /wp:core/heading -->',
 
 		'<!-- wp:core/text -->',
-		'<p>Imagine everything that WordPress can do is available to you quickly and in the same place on the interface. No need to figure out HTML tags, classes, or remember complicated shortcode syntax. That\'s the spirit behind the inserter—the <code>(+)</code> button you\'ll see around the editor—which allows you to browse all available content blocks and insert them into your post. Plugins and themes are able to register their own, opening up all sort of possibilities for rich editing and publishing.</p>',
+		'<p>Imagine everything that WordPress can do is available to you quickly and in the same place on the interface. No need to figure out HTML tags, classes, or remember complicated shortcode syntax. That&#x27;s the spirit behind the inserter—the <code>(+)</code> button you&#x27;ll see around the editor—which allows you to browse all available content blocks and insert them into your post. Plugins and themes are able to register their own, opening up all sort of possibilities for rich editing and publishing.</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/text -->',
-		'<p>Go give it a try, you may discover things WordPress can already insert into your posts that you didn\'t know about. Here\'s a short list of what you can currently find there:</p>',
+		'<p>Go give it a try, you may discover things WordPress can already insert into your posts that you didn&#x27;t know about. Here&#x27;s a short list of what you can currently find there:</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/list -->',
-		'<ul><li>Text & Headings</li><li>Images & Videos</li><li>Galleries</li><li>Embeds, like YouTube, Tweets, or other WordPress posts.</li><li>Layout blocks, like Buttons, Hero Images, Separators, etc.</li><li>And <em>Lists</em> like this one of course :)</li></ul>',
+		'<ul><li>Text &amp; Headings</li><li>Images &amp; Videos</li><li>Galleries</li><li>Embeds, like YouTube, Tweets, or other WordPress posts.</li><li>Layout blocks, like Buttons, Hero Images, Separators, etc.</li><li>And <em>Lists</em> like this one of course :)</li></ul>',
 		'<!-- /wp:core/list -->',
 
 		'<!-- wp:core/text -->',
@@ -69,7 +69,7 @@ window._wpGutenbergPost.content = {
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/button { "align": "center" } -->',
-		'<div class="aligncenter wp-block-button"><a href="https://github.com/WordPress/gutenberg"><span>Help build Gutenberg</span></a></div>',
+		'<div class="wp-block-button aligncenter"><a href="https://github.com/WordPress/gutenberg"><span>Help build Gutenberg</span></a></div>',
 		'<!-- /wp:core/button -->',
 
 		'<!-- wp:core/separator -->',
@@ -84,20 +84,20 @@ window._wpGutenbergPost.content = {
 		'<p>A huge benefit of blocks is that you can edit them in place and manipulate your content directly. Instead of having fields for editing things like the source of a quote, or the text of a button, you can directly change the content. Try editing the following quote:</p>',
 		'<!-- /wp:core/text -->',
 
-		'<!-- wp:core/quote { "style": 1 } -->',
-		'<blockquote class="blocks-quote-style-1 wp-block-quote"><p>The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.</p><footer>Matt Mullenweg, 2017</footer></blockquote>',
+		'<!-- wp:core/quote {"style":1} -->',
+		'<blockquote class="wp-block-quote blocks-quote-style-1"><p>The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.</p><footer>Matt Mullenweg, 2017</footer></blockquote>',
 		'<!-- /wp:core/quote -->',
 
 		'<!-- wp:core/text -->',
-		'<p>The information corresponding to the source of the quote is a separate text field, similar to captions under images, so the structure of the quote is protected even if you select, modify, or remove the source. It\'s always easy to add it back.</p>',
+		'<p>The information corresponding to the source of the quote is a separate text field, similar to captions under images, so the structure of the quote is protected even if you select, modify, or remove the source. It&#x27;s always easy to add it back.</p>',
 		'<!-- /wp:core/text -->',
 
 		'<!-- wp:core/text -->',
 		'<p>Blocks can be anything you need. For instance, you may want to insert a subdued quote as part of the composition of your text, or you may prefer to display a giant stylized one. All of these options are available in the inserter.</p>',
 		'<!-- /wp:core/text -->',
 
-		'<!-- wp:core/quote { "style": 2 } -->',
-		'<blockquote class="blocks-quote-style-2 wp-block-quote"><p>There is no greater agony than bearing an untold story inside you.</p><footer>Maya Angelou</footer></blockquote>',
+		'<!-- wp:core/quote {"style":2} -->',
+		'<blockquote class="wp-block-quote blocks-quote-style-2"><p>There is no greater agony than bearing an untold story inside you.</p><footer>Maya Angelou</footer></blockquote>',
 		'<!-- /wp:core/quote -->',
 
 		'<!-- wp:core/separator -->',
@@ -112,8 +112,8 @@ window._wpGutenbergPost.content = {
 		'<p>Ea veniam homero eam. Ex inimicus molestiae cum, debet scaevola at eos. Vis assum veritus ut, has ea nostrud accusata, offendit appareat comprehensam ea pro. Ad quo quem veritus appellantur, te est quas phaedrum, eum alia habeo ad. Ei est erroribus imperdiet, omnis dicam propriae sed no. His vitae oratio fierent ne, cu duo tota eligendi, electram rationibus in qui.</p>',
 		'<!-- /wp:core/text -->',
 
-		'<!-- wp:core/image { "align": "wide" } -->',
-		'<figure class="wp-block-image"><img alt="Accessibility is important don\'t forget image alt attribute" src="https://cldup.com/uuUqE_dXzy.jpg" /></figure>',
+		'<!-- wp:core/image {"align":"wide"} -->',
+		'<figure class="wp-block-image alignwide"><img src="https://cldup.com/uuUqE_dXzy.jpg" alt="Accessibility is important don&#x27;t forget image alt attribute" /></figure>',
 		'<!-- /wp:core/image -->',
 
 		'<!-- wp:core/text -->',
@@ -128,17 +128,18 @@ window._wpGutenbergPost.content = {
 		'<pre class="wp-block-code"><code>export default function MyButton() {\n\
 return &lt;Button&gt;Click Me!&lt;/Button&gt;;\n\
 }</code></pre>',
+
 		'<!-- /wp:core/code -->',
 
 		'<!-- wp:core/gallery {"align":"wide","images":[{"url":"https://cldup.com/GCwahb3aOb.jpg","alt":""},{"url":"https://cldup.com/lUUQPv6w9c.jpg","alt":""}]} -->',
-		'<div class="blocks-gallery alignwide columns-2 wp-block-gallery">',
+		'<div class="wp-block-gallery alignwide columns-2 is-cropped">',
 		'<figure class="blocks-gallery-image"><img src="https://cldup.com/GCwahb3aOb.jpg" alt="" /></figure>',
 		'<figure class="blocks-gallery-image"><img src="https://cldup.com/lUUQPv6w9c.jpg" alt="" /></figure>',
 		'</div>',
 		'<!-- /wp:core/gallery -->',
 
 		'<!-- wp:core/preformatted -->',
-		'<pre class="wp-block-preformatted">An old silent pond...<br>A frog jumps into the pond,<br>splash! Silence again.</pre>',
+		'<pre class="wp-block-preformatted">An old silent pond...<br/>A frog jumps into the pond,<br/>splash! Silence again.</pre>',
 		'<!-- /wp:core/preformatted -->',
 
 		'<!-- wp:core/text -->',
@@ -154,7 +155,7 @@ return &lt;Button&gt;Click Me!&lt;/Button&gt;;\n\
 		'<!-- /wp:core/list -->',
 
 		'<!-- wp:core/pullquote -->',
-		'<blockquote class="blocks-pullquote wp-block-pullquote"><p>Code is Poetry</p><footer>The WordPress community</footer></blockquote>',
+		'<blockquote class="wp-block-pullquote alignnone"><p>Code is Poetry</p><footer>The WordPress community</footer></blockquote>',
 		'<!-- /wp:core/pullquote -->',
 
 		'<!-- wp:core/separator -->',
@@ -170,16 +171,16 @@ return &lt;Button&gt;Click Me!&lt;/Button&gt;;\n\
 		'<h2>All that you can embed!</h2>',
 		'<!-- /wp:core/heading -->',
 
-		'<!-- wp:core/embed { "url": "https://www.youtube.com/watch?v=Nl6U7UotA-M" } -->',
-		'<figure class="wp-block-embed">https://www.youtube.com/watch?v=Nl6U7UotA-M<figcaption>State of the Word 2016</figcaption></figure>',
+		'<!-- wp:core/embed {"url":"https://www.youtube.com/watch?v=Nl6U7UotA-M"} -->',
+		'<figure class="wp-block-embed">\nhttps://www.youtube.com/watch?v=Nl6U7UotA-M\n<figcaption>State of the Word 2016</figcaption></figure>',
 		'<!-- /wp:core/embed -->',
 
-		'<!-- wp:core/embed { "url": "https://twitter.com/photomatt/status/868657763970404352" } -->',
-		'https://twitter.com/photomatt/status/868657763970404352',
+		'<!-- wp:core/embed {"url":"https://twitter.com/photomatt/status/868657763970404352"} -->',
+		'<figure class="wp-block-embed">\nhttps://twitter.com/photomatt/status/868657763970404352\n</figure>',
 		'<!-- /wp:core/embed -->',
 
-		'<!-- wp:core/embed { "url": "https://make.wordpress.org/core/2017/01/17/editor-technical-overview/" } -->',
-		'https://make.wordpress.org/core/2017/01/17/editor-technical-overview/',
+		'<!-- wp:core/embed {"url":"https://make.wordpress.org/core/2017/01/17/editor-technical-overview/"} -->',
+		'<figure class="wp-block-embed">\nhttps://make.wordpress.org/core/2017/01/17/editor-technical-overview/\n</figure>',
 		'<!-- /wp:core/embed -->',
 
 	].join( '' ),