diff --git a/blocks/api/test/parser.js b/blocks/api/test/parser.js
index 3eb16a2878e59..5ed001bb49940 100644
--- a/blocks/api/test/parser.js
+++ b/blocks/api/test/parser.js
@@ -1,8 +1,3 @@
-/**
- * External dependencies
- */
-import { noop } from 'lodash';
-
 /**
  * Internal dependencies
  */
@@ -32,6 +27,18 @@ describe( 'block parser', () => {
 		title: 'block title',
 	};
 
+	const unknownBlockSettings = {
+		category: 'common',
+		title: 'unknown block',
+		attributes: {
+			content: {
+				type: 'string',
+				source: 'html',
+			},
+		},
+		save: ( { attributes } ) => attributes.content,
+	};
+
 	afterEach( () => {
 		setUnknownTypeHandlerName( undefined );
 		getBlockTypes().forEach( ( block ) => {
@@ -163,7 +170,7 @@ describe( 'block parser', () => {
 
 			const block = createBlockWithFallback(
 				'core/test-block',
-				'content',
+				'Bananas',
 				{ fruit: 'Bananas' }
 			);
 			expect( block.name ).toEqual( 'core/test-block' );
@@ -173,49 +180,35 @@ describe( 'block parser', () => {
 		it( 'should create the requested block with no attributes if it exists', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
 
-			const block = createBlockWithFallback( 'core/test-block', 'content' );
+			const block = createBlockWithFallback( 'core/test-block', '' );
 			expect( block.name ).toEqual( 'core/test-block' );
 			expect( block.attributes ).toEqual( {} );
 		} );
 
 		it( 'should fall back to the unknown type handler for unknown blocks if present', () => {
-			registerBlockType( 'core/unknown-block', {
-				category: 'common',
-				title: 'unknown block',
-				attributes: {
-					content: {
-						type: 'string',
-						source: 'html',
-					},
-					fruit: {
-						type: 'string',
-					},
-				},
-				save: ( { attributes } ) => attributes.content,
-			} );
+			registerBlockType( 'core/unknown-block', unknownBlockSettings );
 			setUnknownTypeHandlerName( 'core/unknown-block' );
 
 			const block = createBlockWithFallback(
 				'core/test-block',
-				'content',
+				'Bananas',
 				{ fruit: 'Bananas' }
 			);
 			expect( block.name ).toBe( 'core/unknown-block' );
-			expect( block.attributes.fruit ).toBe( 'Bananas' );
 			expect( block.attributes.content ).toContain( 'wp:test-block' );
 		} );
 
 		it( 'should fall back to the unknown type handler if block type not specified', () => {
-			registerBlockType( 'core/unknown-block', defaultBlockSettings );
+			registerBlockType( 'core/unknown-block', unknownBlockSettings );
 			setUnknownTypeHandlerName( 'core/unknown-block' );
 
 			const block = createBlockWithFallback( null, 'content' );
 			expect( block.name ).toEqual( 'core/unknown-block' );
-			expect( block.attributes ).toEqual( {} );
+			expect( block.attributes ).toEqual( { content: 'content' } );
 		} );
 
 		it( 'should not create a block if no unknown type handler', () => {
-			const block = createBlockWithFallback( 'core/test-block', 'content' );
+			const block = createBlockWithFallback( 'core/test-block', '' );
 			expect( block ).toBeUndefined();
 		} );
 	} );
@@ -232,7 +225,7 @@ describe( 'block parser', () => {
 					url: { type: 'string' },
 					chicken: { type: 'string' },
 				},
-				save: noop,
+				save: ( { attributes } ) => attributes.content,
 				category: 'common',
 				title: 'test block',
 			} );
@@ -268,7 +261,7 @@ describe( 'block parser', () => {
 						source: 'text',
 					},
 				},
-				save: noop,
+				save: ( { attributes } ) => attributes.content,
 				category: 'common',
 				title: 'test block',
 			} );
@@ -291,7 +284,7 @@ describe( 'block parser', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
 
 			const parsed = parse(
-				'<!-- wp:test-block -->\nRibs\n<!-- /wp:test-block -->'
+				'<!-- wp:test-block {"fruit":"Bananas"} -->\nBananas\n<!-- /wp:test-block -->'
 			);
 
 			expect( parsed ).toHaveLength( 1 );
@@ -304,7 +297,7 @@ describe( 'block parser', () => {
 			setUnknownTypeHandlerName( 'core/unknown-block' );
 
 			const parsed = parse(
-				'<!-- wp:core/test-block -->Ribs<!-- /wp:core/test-block -->' +
+				'<!-- wp:test-block {"fruit":"Bananas"} -->\nBananas\n<!-- /wp:test-block -->' +
 				'<p>Broccoli</p>' +
 				'<!-- wp:core/unknown/block -->Ribs<!-- /wp:core/unknown/block -->'
 			);
@@ -314,12 +307,12 @@ describe( 'block parser', () => {
 
 		it( 'should parse the post content, using unknown block handler', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
-			registerBlockType( 'core/unknown-block', defaultBlockSettings );
+			registerBlockType( 'core/unknown-block', unknownBlockSettings );
 
 			setUnknownTypeHandlerName( 'core/unknown-block' );
 
 			const parsed = parse(
-				'<!-- wp:core/test-block -->Ribs<!-- /wp:core/test-block -->' +
+				'<!-- wp:test-block {"fruit":"Bananas"} -->\nBananas\n<!-- /wp:test-block -->' +
 				'<p>Broccoli</p>' +
 				'<!-- wp:core/unknown-block -->Ribs<!-- /wp:core/unknown-block -->'
 			);
@@ -334,25 +327,15 @@ describe( 'block parser', () => {
 
 		it( 'should parse the post content, including raw HTML at each end', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
-			registerBlockType( 'core/unknown-block', {
-				attributes: {
-					content: {
-						type: 'string',
-						source: 'html',
-					},
-				},
-				save: noop,
-				category: 'common',
-				title: 'unknown block',
-			} );
+			registerBlockType( 'core/unknown-block', unknownBlockSettings );
 
 			setUnknownTypeHandlerName( 'core/unknown-block' );
 
 			const parsed = parse(
 				'<p>Cauliflower</p>' +
-				'<!-- wp:core/test-block -->Ribs<!-- /wp:core/test-block -->' +
+				'<!-- wp:test-block {"fruit":"Bananas"} -->\nBananas\n<!-- /wp:test-block -->' +
 				'\n<p>Broccoli</p>\n' +
-				'<!-- wp:core/test-block -->Ribs<!-- /wp:core/test-block -->' +
+				'<!-- wp:test-block {"fruit":"Bananas"} -->\nBananas\n<!-- /wp:test-block -->' +
 				'<p>Romanesco</p>'
 			);
 
diff --git a/blocks/api/test/validation.js b/blocks/api/test/validation.js
index f203913491685..6e3aabba82169 100644
--- a/blocks/api/test/validation.js
+++ b/blocks/api/test/validation.js
@@ -30,6 +30,18 @@ describe( 'validation', () => {
 		title: 'block title',
 	};
 
+	/* eslint-disable no-console */
+	function expectError() {
+		expect( console.error ).toHaveBeenCalled();
+		console.error.mockClear();
+	}
+
+	function expectWarning() {
+		expect( console.warn ).toHaveBeenCalled();
+		console.warn.mockClear();
+	}
+	/* eslint-enable no-console */
+
 	afterEach( () => {
 		setUnknownTypeHandlerName( undefined );
 		getBlockTypes().forEach( ( block ) => {
@@ -102,6 +114,7 @@ describe( 'validation', () => {
 				{ chars: 'a \n c \t b  ' },
 			);
 
+			expectWarning();
 			expect( isEqual ).toBe( false );
 		} );
 
@@ -178,7 +191,7 @@ describe( 'validation', () => {
 				expect( isEqual ).toBe( true );
 			} );
 
-			it( 'returns true if not same style', () => {
+			it( 'returns false if not same style', () => {
 				const isEqual = isEqualAttributesOfName.style(
 					'background-image: url( "https://wordpress.org/img.png" ); color: red;',
 					'color: red;  font-size: 13px; background-image: url(\'https://wordpress.org/img.png\');'
@@ -201,6 +214,7 @@ describe( 'validation', () => {
 				]
 			);
 
+			expectWarning();
 			expect( isEqual ).toBe( false );
 		} );
 
@@ -228,6 +242,7 @@ describe( 'validation', () => {
 					{ tagName: 'section' }
 				);
 
+				expectWarning();
 				expect( isEqual ).toBe( false );
 			} );
 
@@ -248,6 +263,7 @@ describe( 'validation', () => {
 					}
 				);
 
+				expectWarning();
 				expect( isEqual ).toBe( false );
 			} );
 
@@ -306,6 +322,7 @@ describe( 'validation', () => {
 				'<div>Hello <span class="a">World!</span></div>'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 
@@ -324,6 +341,7 @@ describe( 'validation', () => {
 				'<div>Hello'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 
@@ -333,6 +351,7 @@ describe( 'validation', () => {
 				'<div>Hello</div>'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 
@@ -369,6 +388,7 @@ describe( 'validation', () => {
 				'<input>'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 
@@ -378,6 +398,7 @@ describe( 'validation', () => {
 				'<div>'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 
@@ -387,22 +408,27 @@ describe( 'validation', () => {
 				'<div>'
 			);
 
+			expectWarning();
 			expect( isEquivalent ).toBe( false );
 		} );
 	} );
 
 	describe( 'isValidBlock()', () => {
-		it( 'returns false is block is not valid', () => {
+		it( 'returns false if block is not valid', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
 
-			expect( isValidBlock(
+			const isValid = isValidBlock(
 				'Apples',
 				getBlockType( 'core/test-block' ),
 				{ fruit: 'Bananas' }
-			) ).toBe( false );
+			);
+
+			expectWarning();
+			expectError();
+			expect( isValid ).toBe( false );
 		} );
 
-		it( 'returns false is error occurs while generating block save', () => {
+		it( 'returns false if error occurs while generating block save', () => {
 			registerBlockType( 'core/test-block', {
 				...defaultBlockSettings,
 				save() {
@@ -410,21 +436,26 @@ describe( 'validation', () => {
 				},
 			} );
 
-			expect( isValidBlock(
+			const isValid = isValidBlock(
 				'Bananas',
 				getBlockType( 'core/test-block' ),
 				{ fruit: 'Bananas' }
-			) ).toBe( false );
+			);
+
+			expectError();
+			expect( isValid ).toBe( false );
 		} );
 
 		it( 'returns true is block is valid', () => {
 			registerBlockType( 'core/test-block', defaultBlockSettings );
 
-			expect( isValidBlock(
+			const isValid = isValidBlock(
 				'Bananas',
 				getBlockType( 'core/test-block' ),
 				{ fruit: 'Bananas' }
-			) ).toBe( true );
+			);
+
+			expect( isValid ).toBe( true );
 		} );
 	} );
 } );
diff --git a/blocks/api/validation.js b/blocks/api/validation.js
index cf6c99d8374d8..1ea863c9e341d 100644
--- a/blocks/api/validation.js
+++ b/blocks/api/validation.js
@@ -125,6 +125,36 @@ const MEANINGFUL_ATTRIBUTES = [
 	...ENUMERATED_ATTRIBUTES,
 ];
 
+/**
+ * Object of logger functions.
+ */
+const log = ( () => {
+	/**
+	 * Creates a logger with block validation prefix.
+	 *
+	 * @param  {Function} logger Original logger function
+	 * @return {Function}        Augmented logger function
+	 */
+	function createLogger( logger ) {
+		// In test environments, pre-process the sprintf message to improve
+		// readability of error messages. We'd prefer to avoid pulling in this
+		// dependency in runtime environments, and it can be dropped by a combo
+		// of Webpack env substitution + UglifyJS dead code elimination.
+		if ( process.env.NODE_ENV === 'test' ) {
+			return ( ...args ) => logger( require( 'sprintf-js' ).sprintf( ...args ) );
+		}
+
+		return ( message, ...args ) => logger( 'Block validation: ' + message, ...args );
+	}
+
+	return {
+		/* eslint-disable no-console */
+		error: createLogger( console.error ),
+		warning: createLogger( console.warn ),
+		/* eslint-enable no-console */
+	};
+} )();
+
 /**
  * Given a specified string, returns an array of strings split by consecutive
  * whitespace, ignoring leading or trailing whitespace.
@@ -172,16 +202,22 @@ export function getMeaningfulAttributePairs( token ) {
  * Returns true if two text tokens (with `chars` property) are equivalent, or
  * false otherwise.
  *
- * @param  {Object}  a First token
- * @param  {Object}  b Second token
- * @return {Boolean}   Whether two text tokens are equivalent
+ * @param  {Object}  actual   Actual token
+ * @param  {Object}  expected Expected token
+ * @return {Boolean}          Whether two text tokens are equivalent
  */
-export function isEqualTextTokensWithCollapsedWhitespace( a, b ) {
+export function isEqualTextTokensWithCollapsedWhitespace( actual, expected ) {
 	// This is an overly simplified whitespace comparison. The specification is
 	// more prescriptive of whitespace behavior in inline and block contexts.
 	//
 	// See: https://medium.com/@patrickbrosset/when-does-white-space-matter-in-html-b90e8a7cdd33
-	return isEqual( ...[ a.chars, b.chars ].map( getTextWithCollapsedWhitespace ) );
+	const isEquivalentText = isEqual( ...[ actual.chars, expected.chars ].map( getTextWithCollapsedWhitespace ) );
+
+	if ( ! isEquivalentText ) {
+		log.warning( 'Expected text `%s`, saw `%s`.', expected.chars, actual.chars );
+	}
+
+	return isEquivalentText;
 }
 
 /**
@@ -230,13 +266,13 @@ export function getStyleProperties( text ) {
  * @type {Object}
  */
 export const isEqualAttributesOfName = {
-	class: ( a, b ) => {
+	class: ( actual, expected ) => {
 		// Class matches if members are the same, even if out of order or
 		// superfluous whitespace between.
-		return ! xor( ...[ a, b ].map( getTextPiecesSplitOnWhitespace ) ).length;
+		return ! xor( ...[ actual, expected ].map( getTextPiecesSplitOnWhitespace ) ).length;
 	},
-	style: ( a, b ) => {
-		return isEqual( ...[ a, b ].map( getStyleProperties ) );
+	style: ( actual, expected ) => {
+		return isEqual( ...[ actual, expected ].map( getStyleProperties ) );
 	},
 };
 
@@ -244,38 +280,42 @@ export const isEqualAttributesOfName = {
  * Given two sets of attribute tuples, returns true if the attribute sets are
  * equivalent
  *
- * @param  {Array[]} a First attributes tuples
- * @param  {Array[]} b Second attributes tuples
- * @return {Boolean}   Whether attributes are equivalent
+ * @param  {Array[]} actual   Actual attributes tuples
+ * @param  {Array[]} expected Expected attributes tuples
+ * @return {Boolean}          Whether attributes are equivalent
  */
-export function isEqualTagAttributePairs( a, b ) {
+export function isEqualTagAttributePairs( actual, expected ) {
 	// Attributes is tokenized as tuples. Their lengths should match. This also
 	// avoids us needing to check both attributes sets, since if A has any keys
 	// which do not exist in B, we know the sets to be different.
-	if ( a.length !== b.length ) {
+	if ( actual.length !== expected.length ) {
+		log.warning( 'Expected attributes %o, instead saw %o.', expected, actual );
 		return false;
 	}
 
 	// Convert tuples to object for ease of lookup
-	const [ aAttributes, bAttributes ] = [ a, b ].map( fromPairs );
+	const [ actualAttributes, expectedAttributes ] = [ actual, expected ].map( fromPairs );
 
-	for ( const name in aAttributes ) {
+	for ( const name in actualAttributes ) {
 		// As noted above, if missing member in B, assume different
-		if ( ! bAttributes.hasOwnProperty( name ) ) {
+		if ( ! expectedAttributes.hasOwnProperty( name ) ) {
+			log.warning( 'Encountered unexpected attribute `%s`.', name );
 			return false;
 		}
 
-		const aValue = aAttributes[ name ];
-		const bValue = bAttributes[ name ];
+		const actualValue = actualAttributes[ name ];
+		const expectedValue = expectedAttributes[ name ];
 
 		const isEqualAttributes = isEqualAttributesOfName[ name ];
 		if ( isEqualAttributes ) {
 			// Defer custom attribute equality handling
-			if ( ! isEqualAttributes( aValue, bValue ) ) {
+			if ( ! isEqualAttributes( actualValue, expectedValue ) ) {
+				log.warning( 'Expected attribute `%s` of value `%s`, saw `%s`.', name, expectedValue, actualValue );
 				return false;
 			}
-		} else if ( aValue !== bValue ) {
+		} else if ( actualValue !== expectedValue ) {
 			// Otherwise strict inequality should bail
+			log.warning( 'Expected attribute `%s` of value `%s`, saw `%s`.', name, expectedValue, actualValue );
 			return false;
 		}
 	}
@@ -289,13 +329,14 @@ export function isEqualTagAttributePairs( a, b ) {
  * @type {Object}
  */
 export const isEqualTokensOfType = {
-	StartTag: ( a, b ) => {
-		if ( a.tagName !== b.tagName ) {
+	StartTag: ( actual, expected ) => {
+		if ( actual.tagName !== expected.tagName ) {
+			log.warning( 'Expected tag name `%s`, instead saw `%s`.', actual.tagName, expected.tagName );
 			return false;
 		}
 
 		return isEqualTagAttributePairs(
-			...[ a, b ].map( getMeaningfulAttributePairs )
+			...[ actual, expected ].map( getMeaningfulAttributePairs )
 		);
 	},
 	Chars: isEqualTextTokensWithCollapsedWhitespace,
@@ -328,13 +369,13 @@ export function getNextNonWhitespaceToken( tokens ) {
  * Returns true if there is given HTML strings are effectively equivalent, or
  * false otherwise.
  *
- * @param  {String}  a First HTML string
- * @param  {String}  b Second HTML string
+ * @param  {String}  actual Actual HTML string
+ * @param  {String}  expected Expected HTML string
  * @return {Boolean}   Whether HTML strings are equivalent
  */
-export function isEquivalentHTML( a, b ) {
+export function isEquivalentHTML( actual, expected ) {
 	// Tokenize input content and reserialized save content
-	const [ actualTokens, expectedTokens ] = [ a, b ].map( tokenize );
+	const [ actualTokens, expectedTokens ] = [ actual, expected ].map( tokenize );
 
 	let actualToken, expectedToken;
 	while ( ( actualToken = getNextNonWhitespaceToken( actualTokens ) ) ) {
@@ -342,11 +383,13 @@ export function isEquivalentHTML( a, b ) {
 
 		// Inequal if exhausted all expected tokens
 		if ( ! expectedToken ) {
+			log.warning( 'Expected end of content, instead saw %o.', actualToken );
 			return false;
 		}
 
 		// Inequal if next non-whitespace token of each set are not same type
 		if ( actualToken.type !== expectedToken.type ) {
+			log.warning( 'Expected token of type `%s` (%o), instead saw `%s` (%o).', expectedToken.type, expectedToken, actualToken.type, actualToken );
 			return false;
 		}
 
@@ -361,6 +404,7 @@ export function isEquivalentHTML( a, b ) {
 	while ( ( expectedToken = getNextNonWhitespaceToken( expectedTokens ) ) ) {
 		// If any non-whitespace tokens remain in expected token set, this
 		// indicates inequality
+		log.warning( 'Expected %o, instead saw end of content.', expectedToken );
 		return false;
 	}
 
@@ -384,8 +428,20 @@ export function isValidBlock( innerHTML, blockType, attributes ) {
 	try {
 		saveContent = getSaveContent( blockType, attributes );
 	} catch ( error ) {
+		log.error( 'Block validation failed because an error occurred while generating block content:\n\n%s', error.toString() );
 		return false;
 	}
 
-	return isEquivalentHTML( innerHTML, saveContent );
+	const isValid = isEquivalentHTML( innerHTML, saveContent );
+	if ( ! isValid ) {
+		log.error(
+			'Block validation failed for `%s` (%o).\n\nExpected:\n\n%s\n\nActual:\n\n%s',
+			blockType.name,
+			blockType,
+			saveContent,
+			innerHTML
+		);
+	}
+
+	return isValid;
 }
diff --git a/blocks/test/fixtures/core__gallery.html b/blocks/test/fixtures/core__gallery.html
index 37aff1683b508..b74b304f41e3f 100644
--- a/blocks/test/fixtures/core__gallery.html
+++ b/blocks/test/fixtures/core__gallery.html
@@ -1,5 +1,5 @@
 <!-- wp:core/gallery -->
-<div class="wp-block-gallery">
+<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>
diff --git a/blocks/test/fixtures/core__gallery.json b/blocks/test/fixtures/core__gallery.json
index e10fadd76a9a2..4d1777693f753 100644
--- a/blocks/test/fixtures/core__gallery.json
+++ b/blocks/test/fixtures/core__gallery.json
@@ -2,7 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/gallery",
-        "isValid": false,
+        "isValid": true,
         "attributes": {
             "align": "none",
             "images": [
@@ -18,6 +18,6 @@
             "imageCrop": true,
             "linkTo": "none"
         },
-        "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>"
+        "originalContent": "<div class=\"wp-block-gallery alignnone columns-2 is-cropped\">\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.parsed.json b/blocks/test/fixtures/core__gallery.parsed.json
index f6dcb8c76fa9c..f4c318b6e5213 100644
--- a/blocks/test/fixtures/core__gallery.parsed.json
+++ b/blocks/test/fixtures/core__gallery.parsed.json
@@ -3,7 +3,7 @@
         "blockName": "core/gallery",
         "attrs": null,
         "innerBlocks": [],
-        "innerHTML": "\n<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>\n"
+        "innerHTML": "\n<div class=\"wp-block-gallery alignnone columns-2 is-cropped\">\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>\n"
     },
     {
         "attrs": {},
diff --git a/blocks/test/fixtures/core__gallery.serialized.html b/blocks/test/fixtures/core__gallery.serialized.html
index 57a4f311f52dc..f00d97942ebef 100644
--- a/blocks/test/fixtures/core__gallery.serialized.html
+++ b/blocks/test/fixtures/core__gallery.serialized.html
@@ -1,10 +1,6 @@
 <!-- wp:gallery -->
-<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 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>
 <!-- /wp:gallery -->
diff --git a/blocks/test/fixtures/core__gallery__columns.html b/blocks/test/fixtures/core__gallery__columns.html
index 184fa34783941..3fe69778320f6 100644
--- a/blocks/test/fixtures/core__gallery__columns.html
+++ b/blocks/test/fixtures/core__gallery__columns.html
@@ -1,5 +1,5 @@
 <!-- wp:core/gallery {"columns":"1"} -->
-<div class="wp-block-gallery columns-1 ">
+<div class="wp-block-gallery alignnone columns-1 is-cropped">
 	<figure class="blocks-gallery-image">
 		<img src="https://cldup.com/uuUqE_dXzy.jpg" alt="title" />
 	</figure>
diff --git a/blocks/test/fixtures/core__gallery__columns.json b/blocks/test/fixtures/core__gallery__columns.json
index 7d39a9280edcb..2b3e2e7fca6e6 100644
--- a/blocks/test/fixtures/core__gallery__columns.json
+++ b/blocks/test/fixtures/core__gallery__columns.json
@@ -2,7 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/gallery",
-        "isValid": false,
+        "isValid": true,
         "attributes": {
             "align": "none",
             "images": [
@@ -19,6 +19,6 @@
             "imageCrop": true,
             "linkTo": "none"
         },
-        "originalContent": "<div class=\"wp-block-gallery columns-1 \">\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>"
+        "originalContent": "<div class=\"wp-block-gallery alignnone columns-1 is-cropped\">\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__columns.parsed.json b/blocks/test/fixtures/core__gallery__columns.parsed.json
index 92c888ea61aa6..291420ff0b4c3 100644
--- a/blocks/test/fixtures/core__gallery__columns.parsed.json
+++ b/blocks/test/fixtures/core__gallery__columns.parsed.json
@@ -5,7 +5,7 @@
             "columns": "1"
         },
         "innerBlocks": [],
-        "innerHTML": "\n<div class=\"wp-block-gallery columns-1 \">\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>\n"
+        "innerHTML": "\n<div class=\"wp-block-gallery alignnone columns-1 is-cropped\">\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>\n"
     },
     {
         "attrs": {},
diff --git a/blocks/test/fixtures/core__gallery__columns.serialized.html b/blocks/test/fixtures/core__gallery__columns.serialized.html
index 33822c13f3ef9..39501c3ae1a1e 100644
--- a/blocks/test/fixtures/core__gallery__columns.serialized.html
+++ b/blocks/test/fixtures/core__gallery__columns.serialized.html
@@ -1,10 +1,6 @@
 <!-- wp:gallery {"columns":1} -->
-<div class="wp-block-gallery columns-1 ">
-    <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 alignnone columns-1 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>
 <!-- /wp:gallery -->
diff --git a/blocks/test/fixtures/core__image__center-caption.html b/blocks/test/fixtures/core__image__center-caption.html
index 293d010b4e552..1973035e7c6e6 100644
--- a/blocks/test/fixtures/core__image__center-caption.html
+++ b/blocks/test/fixtures/core__image__center-caption.html
@@ -1,3 +1,3 @@
 <!-- wp:core/image {"align":"center"} -->
-<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>
+<figure class="wp-block-image aligncenter"><img src="https://cldup.com/YLYhpou2oq.jpg"/><figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption></figure>
 <!-- /wp:core/image -->
diff --git a/blocks/test/fixtures/core__image__center-caption.json b/blocks/test/fixtures/core__image__center-caption.json
index 275a384f5f201..c5465d7896235 100644
--- a/blocks/test/fixtures/core__image__center-caption.json
+++ b/blocks/test/fixtures/core__image__center-caption.json
@@ -2,7 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/image",
-        "isValid": false,
+        "isValid": true,
         "attributes": {
             "url": "https://cldup.com/YLYhpou2oq.jpg",
             "caption": [
@@ -10,6 +10,6 @@
             ],
             "align": "center"
         },
-        "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>"
+        "originalContent": "<figure class=\"wp-block-image aligncenter\"><img src=\"https://cldup.com/YLYhpou2oq.jpg\"/><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.parsed.json b/blocks/test/fixtures/core__image__center-caption.parsed.json
index b83c24d3916d6..b32c1f8fd1a0f 100644
--- a/blocks/test/fixtures/core__image__center-caption.parsed.json
+++ b/blocks/test/fixtures/core__image__center-caption.parsed.json
@@ -5,7 +5,7 @@
             "align": "center"
         },
         "innerBlocks": [],
-        "innerHTML": "\n<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>\n"
+        "innerHTML": "\n<figure class=\"wp-block-image aligncenter\"><img src=\"https://cldup.com/YLYhpou2oq.jpg\"/><figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption></figure>\n"
     },
     {
         "attrs": {},
diff --git a/blocks/test/fixtures/core__image__center-caption.serialized.html b/blocks/test/fixtures/core__image__center-caption.serialized.html
index fc8b1f4263ac2..3dffbf76bc841 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:image {"align":"center"} -->
-<figure class="wp-block-image"><img src="https://cldup.com/YLYhpou2oq.jpg" class="aligncenter" />
+<figure class="wp-block-image aligncenter"><img src="https://cldup.com/YLYhpou2oq.jpg" />
     <figcaption>Give it a try. Press the &quot;really wide&quot; button on the image toolbar.</figcaption>
 </figure>
 <!-- /wp:image -->
diff --git a/blocks/test/fixtures/core__pullquote.html b/blocks/test/fixtures/core__pullquote.html
index 82b6234073ab9..8502db91311c1 100644
--- a/blocks/test/fixtures/core__pullquote.html
+++ b/blocks/test/fixtures/core__pullquote.html
@@ -1,5 +1,5 @@
 <!-- wp:core/pullquote -->
-<blockquote class="wp-block-pullquote">
+<blockquote class="wp-block-pullquote alignnone">
 <p>Testing pullquote block...</p><footer>...with a caption</footer>
 </blockquote>
 <!-- /wp:core/pullquote -->
diff --git a/blocks/test/fixtures/core__pullquote.json b/blocks/test/fixtures/core__pullquote.json
index 83c8ec4ec2acf..e264a2bafd908 100644
--- a/blocks/test/fixtures/core__pullquote.json
+++ b/blocks/test/fixtures/core__pullquote.json
@@ -2,7 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/pullquote",
-        "isValid": false,
+        "isValid": true,
         "attributes": {
             "value": [
                 {
@@ -23,6 +23,6 @@
             ],
             "align": "none"
         },
-        "originalContent": "<blockquote class=\"wp-block-pullquote\">\n<p>Testing pullquote block...</p><footer>...with a caption</footer>\n</blockquote>"
+        "originalContent": "<blockquote class=\"wp-block-pullquote alignnone\">\n<p>Testing pullquote block...</p><footer>...with a caption</footer>\n</blockquote>"
     }
 ]
diff --git a/blocks/test/fixtures/core__pullquote.parsed.json b/blocks/test/fixtures/core__pullquote.parsed.json
index ff24bfaa0115b..b2f37bebc98be 100644
--- a/blocks/test/fixtures/core__pullquote.parsed.json
+++ b/blocks/test/fixtures/core__pullquote.parsed.json
@@ -3,7 +3,7 @@
         "blockName": "core/pullquote",
         "attrs": null,
         "innerBlocks": [],
-        "innerHTML": "\n<blockquote class=\"wp-block-pullquote\">\n<p>Testing pullquote block...</p><footer>...with a caption</footer>\n</blockquote>\n"
+        "innerHTML": "\n<blockquote class=\"wp-block-pullquote alignnone\">\n<p>Testing pullquote block...</p><footer>...with a caption</footer>\n</blockquote>\n"
     },
     {
         "attrs": {},
diff --git a/blocks/test/fixtures/core__pullquote.serialized.html b/blocks/test/fixtures/core__pullquote.serialized.html
index 340b4fef74d4c..8dbdbe613ad15 100644
--- a/blocks/test/fixtures/core__pullquote.serialized.html
+++ b/blocks/test/fixtures/core__pullquote.serialized.html
@@ -1,5 +1,5 @@
 <!-- wp:pullquote -->
-<blockquote class="wp-block-pullquote">
+<blockquote class="wp-block-pullquote alignnone">
     <p>Testing pullquote block...</p>
     <footer>...with a caption</footer>
 </blockquote>
diff --git a/blocks/test/fixtures/core__table.html b/blocks/test/fixtures/core__table.html
index 19099e249e5b7..632c38d213f74 100644
--- a/blocks/test/fixtures/core__table.html
+++ b/blocks/test/fixtures/core__table.html
@@ -1,4 +1,4 @@
 <!-- wp:core/table -->
-<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>
+<table class="wp-block-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>
 <!-- /wp:core/table -->
 
diff --git a/blocks/test/fixtures/core__table.json b/blocks/test/fixtures/core__table.json
index ac44c8f471126..2ecdee94dd749 100644
--- a/blocks/test/fixtures/core__table.json
+++ b/blocks/test/fixtures/core__table.json
@@ -2,7 +2,7 @@
     {
         "uid": "_uid_0",
         "name": "core/table",
-        "isValid": false,
+        "isValid": true,
         "attributes": {
             "content": [
                 {
@@ -196,6 +196,6 @@
                 }
             ]
         },
-        "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>"
+        "originalContent": "<table class=\"wp-block-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.parsed.json b/blocks/test/fixtures/core__table.parsed.json
index e5b190a4e983e..0051d503b8d62 100644
--- a/blocks/test/fixtures/core__table.parsed.json
+++ b/blocks/test/fixtures/core__table.parsed.json
@@ -3,7 +3,7 @@
         "blockName": "core/table",
         "attrs": null,
         "innerBlocks": [],
-        "innerHTML": "\n<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>\n"
+        "innerHTML": "\n<table class=\"wp-block-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>\n"
     },
     {
         "attrs": {},
diff --git a/blocks/test/fixtures/core__table.serialized.html b/blocks/test/fixtures/core__table.serialized.html
index 9fe01de807a50..ffa7716b9877d 100644
--- a/blocks/test/fixtures/core__table.serialized.html
+++ b/blocks/test/fixtures/core__table.serialized.html
@@ -1,5 +1,5 @@
 <!-- wp:table -->
-<table>
+<table class="wp-block-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>&hellip;</td>
-            <td>&hellip;</td>
+            <td>…</td>
+            <td>…</td>
         </tr>
         <tr>
             <td><a href="https://wordpress.org/news/2015/12/clifford/">4.4</a></td>
diff --git a/package-lock.json b/package-lock.json
index ae40fb5c2046b..754e39bd3df77 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -248,6 +248,14 @@
       "dev": true,
       "requires": {
         "sprintf-js": "1.0.3"
+      },
+      "dependencies": {
+        "sprintf-js": {
+          "version": "1.0.3",
+          "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.0.3.tgz",
+          "integrity": "sha1-BOaSb2YolTVPPdAVIDYzuFcpfiw=",
+          "dev": true
+        }
       }
     },
     "argv": {
@@ -9391,9 +9399,9 @@
       "integrity": "sha1-yd96NCRZSt5r0RkA1ZZpbcBrrFc="
     },
     "sprintf-js": {
-      "version": "1.0.3",
-      "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.0.3.tgz",
-      "integrity": "sha1-BOaSb2YolTVPPdAVIDYzuFcpfiw=",
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.1.1.tgz",
+      "integrity": "sha1-Nr54Mgr+WAH2zqPueLblqrlA6gw=",
       "dev": true
     },
     "sshpk": {
diff --git a/package.json b/package.json
index 0a35074bfa902..7fa81079f05b7 100644
--- a/package.json
+++ b/package.json
@@ -88,6 +88,7 @@
     "react-test-renderer": "16.0.0",
     "sass-loader": "6.0.6",
     "sass-variables-loader": "0.1.3",
+    "sprintf-js": "1.1.1",
     "style-loader": "0.18.2",
     "tinymce": "4.7.2",
     "webpack": "3.8.1"