Index: /trunk/src/wp-includes/blocks.php
===================================================================
--- /trunk/src/wp-includes/blocks.php	(revision 46895)
+++ /trunk/src/wp-includes/blocks.php	(revision 46896)
@@ -75,9 +75,9 @@
  * @see parse_blocks()
  *
- * @param string                  $block_type Full Block type to look for.
+ * @param string                  $block_name Full Block type to look for.
  * @param int|string|WP_Post|null $post Optional. Post content, post ID, or post object. Defaults to global $post.
  * @return bool Whether the post content contains the specified block.
  */
-function has_block( $block_type, $post = null ) {
+function has_block( $block_name, $post = null ) {
 	if ( ! has_blocks( $post ) ) {
 		return false;
@@ -91,5 +91,28 @@
 	}
 
-	return false !== strpos( $post, '<!-- wp:' . $block_type . ' ' );
+	/*
+	 * Normalize block name to include namespace, if provided as non-namespaced.
+	 * This matches behavior for WordPress 5.0.0 - 5.3.0 in matching blocks by
+	 * their serialized names.
+	 */
+	if ( false === strpos( $block_name, '/' ) ) {
+		$block_name = 'core/' . $block_name;
+	}
+
+	// Test for existence of block by its fully qualified name.
+	$has_block = false !== strpos( $post, '<!-- wp:' . $block_name . ' ' );
+
+	if ( ! $has_block ) {
+		/*
+		 * If the given block name would serialize to a different name, test for
+		 * existence by the serialized form.
+		 */
+		$serialized_block_name = strip_core_block_namespace( $block_name );
+		if ( $serialized_block_name !== $block_name ) {
+			$has_block = false !== strpos( $post, '<!-- wp:' . $serialized_block_name . ' ' );
+		}
+	}
+
+	return $has_block;
 }
 
@@ -112,4 +135,205 @@
 
 	return $dynamic_block_names;
+}
+
+/**
+ * Given an array of attributes, returns a string in the serialized attributes
+ * format prepared for post content.
+ *
+ * The serialized result is a JSON-encoded string, with unicode escape sequence
+ * substitution for characters which might otherwise interfere with embedding
+ * the result in an HTML comment.
+ *
+ * @since 5.3.1
+ *
+ * @param array $attributes Attributes object.
+ * @return string Serialized attributes.
+ */
+function serialize_block_attributes( $block_attributes ) {
+	$encoded_attributes = json_encode( $block_attributes );
+	$encoded_attributes = preg_replace( '/--/', '\\u002d\\u002d', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/</', '\\u003c', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/>/', '\\u003e', $encoded_attributes );
+	$encoded_attributes = preg_replace( '/&/', '\\u0026', $encoded_attributes );
+	// Regex: /\\"/
+	$encoded_attributes = preg_replace( '/\\\\"/', '\\u0022', $encoded_attributes );
+
+	return $encoded_attributes;
+}
+
+/**
+ * Returns the block name to use for serialization. This will remove the default
+ * "core/" namespace from a block name.
+ *
+ * @since 5.3.1
+ *
+ * @param string $block_name Original block name.
+ * @return string Block name to use for serialization.
+ */
+function strip_core_block_namespace( $block_name = null ) {
+	if ( is_string( $block_name ) && 0 === strpos( $block_name, 'core/' ) ) {
+		return substr( $block_name, 5 );
+	}
+
+	return $block_name;
+}
+
+/**
+ * Returns the content of a block, including comment delimiters.
+ *
+ * @since 5.3.1
+ *
+ * @param string $block_name Block name.
+ * @param array  $attributes Block attributes.
+ * @param string $content    Block save content.
+ * @return string Comment-delimited block content.
+ */
+function get_comment_delimited_block_content( $block_name = null, $block_attributes, $block_content ) {
+	if ( is_null( $block_name ) ) {
+		return $block_content;
+	}
+
+	$serialized_block_name = strip_core_block_namespace( $block_name );
+	$serialized_attributes = empty( $block_attributes ) ? '' : serialize_block_attributes( $block_attributes ) . ' ';
+
+	if ( empty( $block_content ) ) {
+		return sprintf( '<!-- wp:%s %s/-->', $serialized_block_name, $serialized_attributes );
+	}
+
+	return sprintf(
+		'<!-- wp:%s %s-->%s<!-- /wp:%s -->',
+		$serialized_block_name,
+		$serialized_attributes,
+		$block_content,
+		$serialized_block_name
+	);
+}
+
+/**
+ * Returns the content of a block, including comment delimiters, serializing all
+ * attributes from the given parsed block.
+ *
+ * This should be used when preparing a block to be saved to post content.
+ * Prefer `render_block` when preparing a block for display. Unlike
+ * `render_block`, this does not evaluate a block's `render_callback`, and will
+ * instead preserve the markup as parsed.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block $block A single parsed block object.
+ * @return string String of rendered HTML.
+ */
+function serialize_block( $block ) {
+	$block_content = '';
+
+	$index = 0;
+	foreach ( $block['innerContent'] as $chunk ) {
+		$block_content .= is_string( $chunk ) ? $chunk : serialize_block( $block['innerBlocks'][ $index++ ] );
+	}
+
+	if ( ! is_array( $block['attrs'] ) ) {
+		$block['attrs'] = array();
+	}
+
+	return get_comment_delimited_block_content(
+		$block['blockName'],
+		$block['attrs'],
+		$block_content
+	);
+}
+
+/**
+ * Returns a joined string of the aggregate serialization of the given parsed
+ * blocks.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block[] $blocks Parsed block objects.
+ * @return string String of rendered HTML.
+ */
+function serialize_blocks( $blocks ) {
+	return implode( '', array_map( 'serialize_block', $blocks ) );
+}
+
+/**
+ * Filters and sanitizes block content to remove non-allowable HTML from
+ * parsed block attribute values.
+ *
+ * @since 5.3.1
+ *
+ * @param string         $text              Text that may contain block content.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return string The filtered and sanitized content result.
+ */
+function filter_block_content( $text, $allowed_html = 'post', $allowed_protocols = array() ) {
+	$result = '';
+
+	$blocks = parse_blocks( $text );
+	foreach ( $blocks as $block ) {
+		$block   = filter_block_kses( $block, $allowed_html, $allowed_protocols );
+		$result .= serialize_block( $block );
+	}
+
+	return $result;
+}
+
+/**
+ * Filters and sanitizes a parsed block to remove non-allowable HTML from block
+ * attribute values.
+ *
+ * @since 5.3.1
+ *
+ * @param WP_Block_Parser_Block $block             The parsed block object.
+ * @param array[]|string        $allowed_html      An array of allowed HTML
+ *                                                 elements and attributes, or a
+ *                                                 context name such as 'post'.
+ * @param string[]              $allowed_protocols Allowed URL protocols.
+ * @return array The filtered and sanitized block object result.
+ */
+function filter_block_kses( $block, $allowed_html, $allowed_protocols = array() ) {
+	$block['attrs'] = filter_block_kses_value( $block['attrs'], $allowed_html, $allowed_protocols );
+
+	if ( is_array( $block['innerBlocks'] ) ) {
+		foreach ( $block['innerBlocks'] as $i => $inner_block ) {
+			$block['innerBlocks'][ $i ] = filter_block_kses( $inner_block, $allowed_html, $allowed_protocols );
+		}
+	}
+
+	return $block;
+}
+
+/**
+ * Filters and sanitizes a parsed block attribute value to remove non-allowable
+ * HTML.
+ *
+ * @since 5.3.1
+ *
+ * @param mixed          $value             The attribute value to filter.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return array The filtered and sanitized result.
+ */
+function filter_block_kses_value( $value, $allowed_html, $allowed_protocols = array() ) {
+	if ( is_array( $value ) ) {
+		foreach ( $value as $key => $inner_value ) {
+			$filtered_key   = filter_block_kses_value( $key, $allowed_html, $allowed_protocols );
+			$filtered_value = filter_block_kses_value( $inner_value, $allowed_html, $allowed_protocols );
+
+			if ( $filtered_key !== $key ) {
+				unset( $value[ $key ] );
+			}
+
+			$value[ $filtered_key ] = $filtered_value;
+		}
+	} elseif ( is_string( $value ) ) {
+		return wp_kses( $value, $allowed_html, $allowed_protocols );
+	}
+
+	return $value;
 }
 
Index: /trunk/src/wp-includes/default-filters.php
===================================================================
--- /trunk/src/wp-includes/default-filters.php	(revision 46895)
+++ /trunk/src/wp-includes/default-filters.php	(revision 46896)
@@ -244,4 +244,5 @@
 add_filter( 'teeny_mce_before_init', '_mce_set_direction' );
 add_filter( 'pre_kses', 'wp_pre_kses_less_than' );
+add_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10, 3 );
 add_filter( 'sanitize_title', 'sanitize_title_with_dashes', 10, 3 );
 add_action( 'check_comment_flood', 'check_comment_flood_db', 10, 4 );
Index: /trunk/src/wp-includes/formatting.php
===================================================================
--- /trunk/src/wp-includes/formatting.php	(revision 46895)
+++ /trunk/src/wp-includes/formatting.php	(revision 46896)
@@ -4906,4 +4906,29 @@
 
 /**
+ * Remove non-allowable HTML from parsed block attribute values when filtering
+ * in the post context.
+ *
+ * @since 5.3.1
+ *
+ * @param string         $string            Content to be run through KSES.
+ * @param array[]|string $allowed_html      An array of allowed HTML elements
+ *                                          and attributes, or a context name
+ *                                          such as 'post'.
+ * @param string[]       $allowed_protocols Array of allowed URL protocols.
+ * @return string Filtered text to run through KSES.
+ */
+function wp_pre_kses_block_attributes( $string, $allowed_html, $allowed_protocols ) {
+	/*
+	 * `filter_block_content` is expected to call `wp_kses`. Temporarily remove
+	 * the filter to avoid recursion.
+	 */
+	remove_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10 );
+	$string = filter_block_content( $string, $allowed_html, $allowed_protocols );
+	add_filter( 'pre_kses', 'wp_pre_kses_block_attributes', 10, 3 );
+
+	return $string;
+}
+
+/**
  * WordPress implementation of PHP sprintf() with filters.
  *
Index: /trunk/tests/phpunit/tests/blocks/block-type.php
===================================================================
--- /trunk/tests/phpunit/tests/blocks/block-type.php	(revision 46895)
+++ /trunk/tests/phpunit/tests/blocks/block-type.php	(revision 46896)
@@ -303,4 +303,22 @@
 		// even if it detects a proper $post global it should still be false for a missing block.
 		$this->assertFalse( has_block( 'core/fake' ) );
+	}
+
+	public function test_post_has_block_serialized_name() {
+		$content = '<!-- wp:serialized /--><!-- wp:core/normalized /--><!-- wp:plugin/third-party /-->';
+
+		$this->assertTrue( has_block( 'core/serialized', $content ) );
+
+		/*
+		 * Technically, `has_block` should receive a "full" (normalized, parsed)
+		 * block name. But this test conforms to expected pre-5.3.1 behavior.
+		 */
+		$this->assertTrue( has_block( 'serialized', $content ) );
+		$this->assertTrue( has_block( 'core/normalized', $content ) );
+		$this->assertTrue( has_block( 'normalized', $content ) );
+		$this->assertFalse( has_block( 'plugin/normalized', $content ) );
+		$this->assertFalse( has_block( 'plugin/serialized', $content ) );
+		$this->assertFalse( has_block( 'third-party', $content ) );
+		$this->assertFalse( has_block( 'core/third-party', $content ) );
 	}
 
