diff --git a/packages/block-library/src/post-excerpt/block.json b/packages/block-library/src/post-excerpt/block.json
index 9809d10ab426b4..61888ced43e67d 100644
--- a/packages/block-library/src/post-excerpt/block.json
+++ b/packages/block-library/src/post-excerpt/block.json
@@ -56,6 +56,18 @@
 		},
 		"interactivity": {
 			"clientNavigation": true
+		},
+		"__experimentalBorder": {
+			"radius": true,
+			"color": true,
+			"width": true,
+			"style": true,
+			"__experimentalDefaultControls": {
+				"radius": true,
+				"color": true,
+				"width": true,
+				"style": true
+			}
 		}
 	},
 	"editorStyle": "wp-block-post-excerpt-editor",
diff --git a/packages/block-library/src/post-excerpt/style.scss b/packages/block-library/src/post-excerpt/style.scss
index 1562bca962c1c9..b44108f67b5dd5 100644
--- a/packages/block-library/src/post-excerpt/style.scss
+++ b/packages/block-library/src/post-excerpt/style.scss
@@ -1,5 +1,8 @@
 // Lowest specificity on wrapper margins to avoid overriding layout styles.
 :where(.wp-block-post-excerpt) {
+	// This block has customizable padding, border-box makes that more predictable.
+	box-sizing: border-box;
+
 	margin-top: var(--wp--style--block-gap);
 	margin-bottom: var(--wp--style--block-gap);
 }