diff --git a/packages/block-editor/src/components/iframe/index.js b/packages/block-editor/src/components/iframe/index.js
index 8ec4b24106ebf3..cc99464f9bb1a8 100644
--- a/packages/block-editor/src/components/iframe/index.js
+++ b/packages/block-editor/src/components/iframe/index.js
@@ -131,8 +131,23 @@ function Iframe( {
 		function preventFileDropDefault( event ) {
 			event.preventDefault();
 		}
+
+		const { ownerDocument } = node;
+
+		// Ideally ALL classes that are added through get_body_class should
+		// be added in the editor too, which we'll somehow have to get from
+		// the server in the future (which will run the PHP filters).
+		setBodyClasses(
+			Array.from( ownerDocument.body.classList ).filter(
+				( name ) =>
+					name.startsWith( 'admin-color-' ) ||
+					name.startsWith( 'post-type-' ) ||
+					name === 'wp-embed-responsive'
+			)
+		);
+
 		function onLoad() {
-			const { contentDocument, ownerDocument } = node;
+			const { contentDocument } = node;
 			const { documentElement } = contentDocument;
 			iFrameDocument = contentDocument;
 
@@ -140,18 +155,6 @@ function Iframe( {
 
 			clearerRef( documentElement );
 
-			// Ideally ALL classes that are added through get_body_class should
-			// be added in the editor too, which we'll somehow have to get from
-			// the server in the future (which will run the PHP filters).
-			setBodyClasses(
-				Array.from( ownerDocument.body.classList ).filter(
-					( name ) =>
-						name.startsWith( 'admin-color-' ) ||
-						name.startsWith( 'post-type-' ) ||
-						name === 'wp-embed-responsive'
-				)
-			);
-
 			contentDocument.dir = ownerDocument.dir;
 
 			for ( const compatStyle of getCompatibilityStyles() ) {