Proof of concept of adding CSS classes to all blocks with the classNa…

…me supports set to true

docs/reference-guides/core-blocks.md

@@ -437,7 +437,7 @@ Start with the basic building block of all narrative. ([Source](https://github.c
 
 -	**Name:** core/paragraph
 -	**Category:** text
--	**Supports:** __unstablePasteTextInline, anchor, color (background, link, text), typography (fontSize, lineHeight), ~~className~~
+-	**Supports:** __unstablePasteTextInline, anchor, className, color (background, link, text), typography (fontSize, lineHeight)
 -	**Attributes:** align, content, direction, dropCap, placeholder
 
 ## Pattern

lib/blocks.php

@@ -23,6 +23,7 @@ function gutenberg_reregister_core_block_types() {
 				'columns',
 				'comments',
 				'group',
+				'heading',
 				'html',
 				'list',
 				'media-text',
@@ -66,7 +67,6 @@ function gutenberg_reregister_core_block_types() {
 				'home-link.php'                    => 'core/home-link',
 				'image.php'                        => 'core/image',
 				'gallery.php'                      => 'core/gallery',
-				'heading.php'                      => 'core/heading',
 				'latest-comments.php'              => 'core/latest-comments',
 				'latest-posts.php'                 => 'core/latest-posts',
 				'loginout.php'                     => 'core/loginout',

lib/experimental/blocks.php

@@ -28,3 +28,89 @@ function gutenberg_enable_experimental_blocks() {
 }
 
 add_action( 'admin_init', 'gutenberg_enable_experimental_blocks' );
+
+function gutenberg_add_default_class_name_to_rendered_block_markup( $block_content, $parsed_block, $block ) {
+	if ( ! $block_content ) {
+		return $block_content;
+	}
+
+	if ( ! $block->block_type || $block->block_type->is_dynamic() ) {
+		return $block_content;
+	}
+
+	$class_name_supports = _wp_array_get( $block->block_type->supports, array( 'className' ), false );
+	if ( ! $class_name_supports ) {
+		return $block_content;
+	}
+
+	return gutenberg_add_class_name_to_wrapping_tag(
+		$block_content,
+		wp_get_block_default_classname( $block->name )
+	);
+}
+
+add_action( 'render_block', 'gutenberg_add_default_class_name_to_rendered_block_markup', 10, 3 );
+
+function gutenberg_add_class_name_to_wrapping_tag( $block_content, $added_class_name ) {
+	$matches = array();
+	$pattern = '/
+    ^(\s*)                               # Any leading whitespace.
+    <(?<tag_name>
+        [a-zA-Z0-9\-]+                   # The opening tag...
+        (?=\s|>)                         # ...followed by a whitespace or >
+                                         # ?= means a "lookahead"
+    )
+    (?<before_class>                     # Any attributes prior to "class"
+        (?:                              # ?: is a "non-capturing group"
+            (?!\sclass=")[^>]            # Match all characters until ">" except when
+                                         # the next character sequence is \sclass="
+                                         # ?! is a "negative lookahead"
+        )*
+    )
+    (?:\s*class="(?<class_name>[^"]+)")? # The class attribute, if any
+    (?<after_class>[^>]*?)               # The rest of the tag
+    >                                    # The closing tag
+/xm';
+
+	preg_match(
+		$pattern,
+		$block_content,
+		$matches
+	);
+
+	if ( empty( $matches ) ) {
+		return $block_content;
+	}
+
+	// Parse the existing class names.
+	$current_class_attribute = ! empty( $matches['class_name'] ) ? $matches['class_name'] : '';
+	$current_class_names     = explode( ' ', $current_class_attribute );
+	$current_class_names     = array_map( 'trim', $current_class_names );
+	$current_class_names     = array_filter( $current_class_names );
+
+	// If wp-block-heading is already included, there's no need to add it again.
+	if ( in_array( $added_class_name, $current_class_names, true ) ) {
+		return $block_content;
+	}
+
+	// Otherwise, let's add it to the class names.
+	$current_class_names[] = $added_class_name;
+	$new_class_attribute   = implode( ' ', $current_class_names );
+
+	// Construct a new opening tag.
+	$new_tag_parts = array(
+		$matches['tag_name'],
+		$matches['before_class'],
+		'class="' . $new_class_attribute . '"',
+		$matches['after_class'],
+	);
+	$new_tag_parts = array_map( 'trim', $new_tag_parts );
+	$new_tag_parts = array_filter( $new_tag_parts );
+	$new_tag       = '<' . implode( ' ', $new_tag_parts ) . '>';
+
+	// Replace the old opening tag with the new one.
+	$initial_whitespace = $matches[1];
+	$old_tag_ends_at    = strlen( $matches[0] );
+
+	return $initial_whitespace . $new_tag . substr( $block_content, $old_tag_ends_at );
+}

packages/block-library/src/paragraph/block.json

@@ -32,7 +32,7 @@
 	},
 	"supports": {
 		"anchor": true,
-		"className": false,
+		"className": true,
 		"color": {
 			"link": true,
 			"__experimentalDefaultControls": {