From e1dbb38e9b58caf2524409341599611bc1e8709a Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Fri, 23 Jan 2026 12:53:33 +1300
Subject: [PATCH 01/12] Add files to backport
 https://github.com/WordPress/gutenberg/pull/73959

---
 src/wp-includes/block-editor.php              |  2 +
 src/wp-includes/block-supports/custom-css.php | 99 +++++++++++++++++++
 src/wp-includes/class-wp-theme-json.php       |  3 +-
 src/wp-settings.php                           |  1 +
 4 files changed, 104 insertions(+), 1 deletion(-)
 create mode 100644 src/wp-includes/block-supports/custom-css.php

diff --git a/src/wp-includes/block-editor.php b/src/wp-includes/block-editor.php
index af873178eb7aa..9d0cfc1cec9f0 100644
--- a/src/wp-includes/block-editor.php
+++ b/src/wp-includes/block-editor.php
@@ -658,6 +658,8 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 		$editor_settings = apply_filters_deprecated( 'block_editor_settings', array( $editor_settings, $post ), '5.8.0', 'block_editor_settings_all' );
 	}
 
+	$settings['canEditCSS'] = current_user_can( 'edit_css' );
+	
 	return $editor_settings;
 }
 
diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
new file mode 100644
index 0000000000000..509372ecef41f
--- /dev/null
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -0,0 +1,99 @@
+<?php
+/**
+ * Custom CSS block support.
+ *
+ * @package gutenberg
+ */
+
+/**
+ * Render the custom CSS stylesheet and add class name to block as required.
+ *
+ * @since 7.0.0
+ *
+ * @param array $parsed_block The parsed block.
+ * @return array The same parsed block with custom CSS class name added if appropriate.
+ */
+function gutenberg_render_custom_css_support_styles( $parsed_block ) {
+	$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $parsed_block['blockName'] );
+
+	if ( ! block_has_support( $block_type, 'customCSS', true ) ) {
+		return $parsed_block;
+	}
+
+	$custom_css = trim( $parsed_block['attrs']['style']['css'] ?? '' );
+
+	if ( empty( $custom_css ) ) {
+		return $parsed_block;
+	}
+
+	// Validate CSS doesn't contain HTML markup (same validation as global styles REST API).
+	if ( preg_match( '#</?\w+#', $custom_css ) ) {
+		return $parsed_block;
+	}
+
+	// Generate a unique class name for this block instance.
+	$class_name         = wp_unique_id_from_values( $parsed_block, 'wp-custom-css-' );
+	$updated_class_name = isset( $parsed_block['attrs']['className'] )
+		? $parsed_block['attrs']['className'] . " $class_name"
+		: $class_name;
+
+	_wp_array_set( $parsed_block, array( 'attrs', 'className' ), $updated_class_name );
+
+	// Process the custom CSS using the same method as global styles.
+	$selector      = '.' . $class_name;
+	$processed_css = WP_Theme_JSON_Gutenberg::process_blocks_custom_css( $custom_css, $selector );
+
+	if ( ! empty( $processed_css ) ) {
+		/*
+		 * Register and add inline style for block custom CSS.
+		 * The style depends on global-styles to ensure custom CSS loads after
+		 * and can override global styles.
+		 */
+		wp_register_style( 'wp-block-custom-css', false, array( 'global-styles' ) );
+		wp_add_inline_style( 'wp-block-custom-css', $processed_css );
+	}
+
+	return $parsed_block;
+}
+
+/**
+ * Enqueues the block custom CSS styles.
+ *
+ * @since 7.0.0
+ */
+function gutenberg_enqueue_block_custom_css() {
+	wp_enqueue_style( 'wp-block-custom-css' );
+}
+
+/**
+ * Applies the custom CSS class name to the block's rendered HTML.
+ *
+ * The class name is generated in `gutenberg_render_custom_css_support_styles`
+ * and stored in block attributes. This filter adds it to the actual markup.
+ *
+ * @since 7.0.0
+ *
+ * @param string $block_content Rendered block content.
+ * @param array  $block         Block object.
+ * @return string               Filtered block content.
+ */
+function gutenberg_render_custom_css_class_name( $block_content, $block ) {
+	$class_string = $block['attrs']['className'] ?? '';
+	preg_match( '/\bwp-custom-css-\S+\b/', $class_string, $matches );
+
+	if ( empty( $matches ) ) {
+		return $block_content;
+	}
+
+	$tags = new WP_HTML_Tag_Processor( $block_content );
+
+	if ( $tags->next_tag() ) {
+		$tags->add_class( $matches[0] );
+	}
+
+	return $tags->get_updated_html();
+}
+
+add_filter( 'render_block', 'gutenberg_render_custom_css_class_name', 10, 2 );
+add_filter( 'render_block_data', 'gutenberg_render_custom_css_support_styles', 10, 1 );
+add_action( 'wp_enqueue_scripts', 'gutenberg_enqueue_block_custom_css', 1 );
diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index c7929dad9e689..cf657364ebb93 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -1520,12 +1520,13 @@ public function get_stylesheet( $types = array( 'variables', 'styles', 'presets'
 	 *
 	 * @since 6.2.0
 	 * @since 6.6.0 Enforced 0-1-0 specificity for block custom CSS selectors.
+	 * @since 7.0.0 Made public for use in custom-css block support.
 	 *
 	 * @param string $css      The CSS to process.
 	 * @param string $selector The selector to nest.
 	 * @return string The processed CSS.
 	 */
-	protected function process_blocks_custom_css( $css, $selector ) {
+	public function process_blocks_custom_css( $css, $selector ) {
 		$processed_css = '';
 
 		if ( empty( $css ) ) {
diff --git a/src/wp-settings.php b/src/wp-settings.php
index 60c220100f539..a509c37f5e51c 100644
--- a/src/wp-settings.php
+++ b/src/wp-settings.php
@@ -412,6 +412,7 @@
 require ABSPATH . WPINC . '/block-supports/aria-label.php';
 require ABSPATH . WPINC . '/block-supports/anchor.php';
 require ABSPATH . WPINC . '/block-supports/block-visibility.php';
+require ABSPATH . WPINC . '/block-supports/custom-css.php';
 require ABSPATH . WPINC . '/style-engine.php';
 require ABSPATH . WPINC . '/style-engine/class-wp-style-engine.php';
 require ABSPATH . WPINC . '/style-engine/class-wp-style-engine-css-declarations.php';

From 28e3da25fda23a96673f1f281fa0b1bc121103ea Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Fri, 23 Jan 2026 13:04:53 +1300
Subject: [PATCH 02/12] Add test

---
 .../tests/block-supports/custom-css.php       | 441 ++++++++++++++++++
 1 file changed, 441 insertions(+)
 create mode 100644 tests/phpunit/tests/block-supports/custom-css.php

diff --git a/tests/phpunit/tests/block-supports/custom-css.php b/tests/phpunit/tests/block-supports/custom-css.php
new file mode 100644
index 0000000000000..c94b5ff7c0611
--- /dev/null
+++ b/tests/phpunit/tests/block-supports/custom-css.php
@@ -0,0 +1,441 @@
+<?php
+/**
+ * Test the custom CSS block support.
+ *
+ * @package gutenberg
+ */
+
+class WP_Block_Supports_Custom_CSS_Test extends WP_UnitTestCase {
+	/**
+	 * @var string|null
+	 */
+	private $test_block_name;
+
+	public function set_up() {
+		parent::set_up();
+		$this->test_block_name = null;
+	}
+
+	public function tear_down() {
+		if ( $this->test_block_name ) {
+			unregister_block_type( $this->test_block_name );
+		}
+		$this->test_block_name = null;
+		parent::tear_down();
+	}
+
+	/**
+	 * Registers a new block for testing custom CSS support.
+	 *
+	 * @param string $block_name Name for the test block.
+	 * @param array  $supports   Array defining block support configuration.
+	 *
+	 * @return WP_Block_Type The block type for the newly registered test block.
+	 */
+	private function register_custom_css_block_with_support( $block_name, $supports = array() ) {
+		$this->test_block_name = $block_name;
+		register_block_type(
+			$this->test_block_name,
+			array(
+				'api_version' => 3,
+				'attributes'  => array(
+					'style' => array(
+						'type' => 'object',
+					),
+				),
+				'supports'    => $supports,
+			)
+		);
+		$registry = WP_Block_Type_Registry::get_instance();
+
+		return $registry->get_registered( $this->test_block_name );
+	}
+
+	/**
+	 * Tests that custom CSS support adds class name when block has custom CSS.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_adds_class_name_when_css_present() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-block',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-block',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'color: red;',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added.' );
+		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should contain wp-custom-css- prefix.' );
+	}
+
+	/**
+	 * Tests that custom CSS support preserves existing className.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_preserves_existing_class_name() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-block-existing',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-block-existing',
+			'attrs'     => array(
+				'className' => 'my-existing-class',
+				'style'     => array(
+					'css' => 'color: blue;',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertStringContainsString( 'my-existing-class', $result['attrs']['className'], 'Existing className should be preserved.' );
+		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should also contain wp-custom-css- prefix.' );
+	}
+
+	/**
+	 * Tests that custom CSS support returns unchanged block when support is disabled.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_returns_unchanged_when_support_disabled() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-disabled',
+			array( 'customCSS' => false )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-disabled',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'color: green;',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when support is disabled.' );
+	}
+
+	/**
+	 * Tests that custom CSS support returns unchanged block when no CSS attribute present.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_returns_unchanged_when_no_css() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-no-css',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-no-css',
+			'attrs'     => array(
+				'style' => array(
+					'color' => 'red',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no CSS attribute.' );
+	}
+
+	/**
+	 * Tests that custom CSS support returns unchanged block when CSS is empty.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_returns_unchanged_when_css_empty() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-empty',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-empty',
+			'attrs'     => array(
+				'style' => array(
+					'css' => '',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is empty.' );
+	}
+
+	/**
+	 * Tests that custom CSS support returns unchanged block when CSS is whitespace only.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_returns_unchanged_when_css_whitespace_only() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-whitespace',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-whitespace',
+			'attrs'     => array(
+				'style' => array(
+					'css' => '   ',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is whitespace only.' );
+	}
+
+	/**
+	 * Tests that custom CSS support returns unchanged block when style attribute is missing.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_returns_unchanged_when_no_style_attribute() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-no-style',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-no-style',
+			'attrs'     => array(),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no style attribute.' );
+	}
+
+	/**
+	 * Tests that render_block filter adds custom CSS class to block content.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_adds_class_to_content() {
+		$block_content = '<div class="wp-block-paragraph">Test content</div>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'wp-custom-css-123abc',
+			),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertStringContainsString( 'wp-custom-css-123abc', $result, 'Custom CSS class should be added to block content.' );
+	}
+
+	/**
+	 * Tests that render_block filter preserves existing classes when adding custom CSS class.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_preserves_existing_classes() {
+		$block_content = '<div class="existing-class another-class">Test content</div>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'wp-custom-css-456def',
+			),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertStringContainsString( 'existing-class', $result, 'Existing classes should be preserved.' );
+		$this->assertStringContainsString( 'another-class', $result, 'All existing classes should be preserved.' );
+		$this->assertStringContainsString( 'wp-custom-css-456def', $result, 'Custom CSS class should be added.' );
+	}
+
+	/**
+	 * Tests that render_block filter returns unchanged content when no custom CSS class in attrs.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_returns_unchanged_when_no_custom_css_class() {
+		$block_content = '<div class="wp-block-paragraph">Test content</div>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'some-other-class',
+			),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when no custom CSS class.' );
+	}
+
+	/**
+	 * Tests that render_block filter returns unchanged content when className is empty.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_returns_unchanged_when_classname_empty() {
+		$block_content = '<div class="wp-block-paragraph">Test content</div>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when className is empty.' );
+	}
+
+	/**
+	 * Tests that render_block filter returns empty string when content is empty.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_returns_empty_when_content_empty() {
+		$block_content = '';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'wp-custom-css-789ghi',
+			),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertSame( '', $result, 'Result should be empty when block content is empty.' );
+	}
+
+	/**
+	 * Tests that custom CSS class is extracted correctly when mixed with other classes.
+	 *
+	 * @covers ::gutenberg_render_custom_css_class_name
+	 */
+	public function test_render_custom_css_class_name_extracts_class_from_mixed_classnames() {
+		$block_content = '<p>Test content</p>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'my-class wp-custom-css-mixed123 another-class',
+			),
+		);
+
+		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertStringContainsString( 'wp-custom-css-mixed123', $result, 'Custom CSS class should be extracted and added.' );
+	}
+
+	/**
+	 * Tests that custom CSS support is enabled by default.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_support_enabled_by_default() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-default',
+			array() // No explicit customCSS support defined.
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-default',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'font-weight: bold;',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added by default when customCSS support is not explicitly set.' );
+	}
+
+	/**
+	 * Tests that custom CSS containing HTML opening tags is rejected.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_rejects_html_opening_tags() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-html-open',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-html-open',
+			'attrs'     => array(
+				'style' => array(
+					'css' => '<script>alert(1)</script>',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML opening tags.' );
+	}
+
+	/**
+	 * Tests that custom CSS containing HTML closing tags is rejected.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_rejects_html_closing_tags() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-html-close',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-html-close',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'color: red;</style><script>alert(1)</script>',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML closing tags.' );
+	}
+
+	/**
+	 * Tests that valid CSS without HTML markup is accepted.
+	 *
+	 * @covers ::gutenberg_render_custom_css_support_styles
+	 */
+	public function test_custom_css_accepts_valid_css() {
+		$this->register_custom_css_block_with_support(
+			'test/custom-css-valid',
+			array( 'customCSS' => true )
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-valid',
+			'attrs'     => array(
+				'style' => array(
+					'css' => 'color: red; background: url("image.png"); font-size: 16px;',
+				),
+			),
+		);
+
+		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added for valid CSS.' );
+	}
+}

From 8646524545e00d23224ebe48426773c2a9d5bd62 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Fri, 23 Jan 2026 13:11:17 +1300
Subject: [PATCH 03/12] lint fix

---
 src/wp-includes/block-editor.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/wp-includes/block-editor.php b/src/wp-includes/block-editor.php
index 9d0cfc1cec9f0..0d9e3c0c1e067 100644
--- a/src/wp-includes/block-editor.php
+++ b/src/wp-includes/block-editor.php
@@ -659,7 +659,7 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 	}
 
 	$settings['canEditCSS'] = current_user_can( 'edit_css' );
-	
+
 	return $editor_settings;
 }
 

From 45a49735f444bd40c6038d44a278f5fcb3308231 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Thu, 12 Feb 2026 15:21:10 +1300
Subject: [PATCH 04/12] fix package name

---
 src/wp-includes/block-supports/custom-css.php     | 2 +-
 tests/phpunit/tests/block-supports/custom-css.php | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 509372ecef41f..608c5055d894e 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -2,7 +2,7 @@
 /**
  * Custom CSS block support.
  *
- * @package gutenberg
+ * @package WordPress
  */
 
 /**
diff --git a/tests/phpunit/tests/block-supports/custom-css.php b/tests/phpunit/tests/block-supports/custom-css.php
index c94b5ff7c0611..5a083d3662f31 100644
--- a/tests/phpunit/tests/block-supports/custom-css.php
+++ b/tests/phpunit/tests/block-supports/custom-css.php
@@ -2,7 +2,7 @@
 /**
  * Test the custom CSS block support.
  *
- * @package gutenberg
+ * @package WordPress
  */
 
 class WP_Block_Supports_Custom_CSS_Test extends WP_UnitTestCase {

From d00dd3362a3e226bc4ad16f9f45bb042de5cc530 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Thu, 12 Feb 2026 15:40:54 +1300
Subject: [PATCH 05/12] add change from
 https://github.com/WordPress/gutenberg/pull/74969

---
 src/wp-includes/block-supports/custom-css.php | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 608c5055d894e..28c31f8cf4d95 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -88,6 +88,7 @@ function gutenberg_render_custom_css_class_name( $block_content, $block ) {
 	$tags = new WP_HTML_Tag_Processor( $block_content );
 
 	if ( $tags->next_tag() ) {
+		$tags->add_class( 'has-custom-css' );
 		$tags->add_class( $matches[0] );
 	}
 

From 6ed1b3bbd5fb343355e6d6c06732138abb2116c3 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Mon, 16 Feb 2026 12:55:58 +1300
Subject: [PATCH 06/12] fix tests

---
 src/wp-includes/block-supports/custom-css.php | 16 ++---
 .../tests/block-supports/custom-css.php       | 68 +++++++++----------
 2 files changed, 42 insertions(+), 42 deletions(-)

diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 28c31f8cf4d95..9a6ef1f4a34e7 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -13,7 +13,7 @@
  * @param array $parsed_block The parsed block.
  * @return array The same parsed block with custom CSS class name added if appropriate.
  */
-function gutenberg_render_custom_css_support_styles( $parsed_block ) {
+function wp_render_custom_css_support_styles( $parsed_block ) {
 	$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $parsed_block['blockName'] );
 
 	if ( ! block_has_support( $block_type, 'customCSS', true ) ) {
@@ -41,7 +41,7 @@ function gutenberg_render_custom_css_support_styles( $parsed_block ) {
 
 	// Process the custom CSS using the same method as global styles.
 	$selector      = '.' . $class_name;
-	$processed_css = WP_Theme_JSON_Gutenberg::process_blocks_custom_css( $custom_css, $selector );
+	$processed_css = WP_Theme_JSON::process_blocks_custom_css( $custom_css, $selector );
 
 	if ( ! empty( $processed_css ) ) {
 		/*
@@ -61,14 +61,14 @@ function gutenberg_render_custom_css_support_styles( $parsed_block ) {
  *
  * @since 7.0.0
  */
-function gutenberg_enqueue_block_custom_css() {
+function wp_enqueue_block_custom_css() {
 	wp_enqueue_style( 'wp-block-custom-css' );
 }
 
 /**
  * Applies the custom CSS class name to the block's rendered HTML.
  *
- * The class name is generated in `gutenberg_render_custom_css_support_styles`
+ * The class name is generated in `wp_render_custom_css_support_styles`
  * and stored in block attributes. This filter adds it to the actual markup.
  *
  * @since 7.0.0
@@ -77,7 +77,7 @@ function gutenberg_enqueue_block_custom_css() {
  * @param array  $block         Block object.
  * @return string               Filtered block content.
  */
-function gutenberg_render_custom_css_class_name( $block_content, $block ) {
+function wp_render_custom_css_class_name( $block_content, $block ) {
 	$class_string = $block['attrs']['className'] ?? '';
 	preg_match( '/\bwp-custom-css-\S+\b/', $class_string, $matches );
 
@@ -95,6 +95,6 @@ function gutenberg_render_custom_css_class_name( $block_content, $block ) {
 	return $tags->get_updated_html();
 }
 
-add_filter( 'render_block', 'gutenberg_render_custom_css_class_name', 10, 2 );
-add_filter( 'render_block_data', 'gutenberg_render_custom_css_support_styles', 10, 1 );
-add_action( 'wp_enqueue_scripts', 'gutenberg_enqueue_block_custom_css', 1 );
+add_filter( 'render_block', 'wp_render_custom_css_class_name', 10, 2 );
+add_filter( 'render_block_data', 'wp_render_custom_css_support_styles', 10, 1 );
+add_action( 'wp_enqueue_scripts', 'wp_enqueue_block_custom_css', 1 );
diff --git a/tests/phpunit/tests/block-supports/custom-css.php b/tests/phpunit/tests/block-supports/custom-css.php
index 5a083d3662f31..b69cc0379559c 100644
--- a/tests/phpunit/tests/block-supports/custom-css.php
+++ b/tests/phpunit/tests/block-supports/custom-css.php
@@ -54,7 +54,7 @@ private function register_custom_css_block_with_support( $block_name, $supports
 	/**
 	 * Tests that custom CSS support adds class name when block has custom CSS.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_adds_class_name_when_css_present() {
 		$this->register_custom_css_block_with_support(
@@ -71,7 +71,7 @@ public function test_custom_css_support_adds_class_name_when_css_present() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added.' );
 		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should contain wp-custom-css- prefix.' );
@@ -80,7 +80,7 @@ public function test_custom_css_support_adds_class_name_when_css_present() {
 	/**
 	 * Tests that custom CSS support preserves existing className.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_preserves_existing_class_name() {
 		$this->register_custom_css_block_with_support(
@@ -98,7 +98,7 @@ public function test_custom_css_support_preserves_existing_class_name() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertStringContainsString( 'my-existing-class', $result['attrs']['className'], 'Existing className should be preserved.' );
 		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should also contain wp-custom-css- prefix.' );
@@ -107,7 +107,7 @@ public function test_custom_css_support_preserves_existing_class_name() {
 	/**
 	 * Tests that custom CSS support returns unchanged block when support is disabled.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_returns_unchanged_when_support_disabled() {
 		$this->register_custom_css_block_with_support(
@@ -124,7 +124,7 @@ public function test_custom_css_support_returns_unchanged_when_support_disabled(
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when support is disabled.' );
 	}
@@ -132,7 +132,7 @@ public function test_custom_css_support_returns_unchanged_when_support_disabled(
 	/**
 	 * Tests that custom CSS support returns unchanged block when no CSS attribute present.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_returns_unchanged_when_no_css() {
 		$this->register_custom_css_block_with_support(
@@ -149,7 +149,7 @@ public function test_custom_css_support_returns_unchanged_when_no_css() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no CSS attribute.' );
 	}
@@ -157,7 +157,7 @@ public function test_custom_css_support_returns_unchanged_when_no_css() {
 	/**
 	 * Tests that custom CSS support returns unchanged block when CSS is empty.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_returns_unchanged_when_css_empty() {
 		$this->register_custom_css_block_with_support(
@@ -174,7 +174,7 @@ public function test_custom_css_support_returns_unchanged_when_css_empty() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is empty.' );
 	}
@@ -182,7 +182,7 @@ public function test_custom_css_support_returns_unchanged_when_css_empty() {
 	/**
 	 * Tests that custom CSS support returns unchanged block when CSS is whitespace only.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_returns_unchanged_when_css_whitespace_only() {
 		$this->register_custom_css_block_with_support(
@@ -199,7 +199,7 @@ public function test_custom_css_support_returns_unchanged_when_css_whitespace_on
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is whitespace only.' );
 	}
@@ -207,7 +207,7 @@ public function test_custom_css_support_returns_unchanged_when_css_whitespace_on
 	/**
 	 * Tests that custom CSS support returns unchanged block when style attribute is missing.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_returns_unchanged_when_no_style_attribute() {
 		$this->register_custom_css_block_with_support(
@@ -220,7 +220,7 @@ public function test_custom_css_support_returns_unchanged_when_no_style_attribut
 			'attrs'     => array(),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no style attribute.' );
 	}
@@ -228,7 +228,7 @@ public function test_custom_css_support_returns_unchanged_when_no_style_attribut
 	/**
 	 * Tests that render_block filter adds custom CSS class to block content.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @@ticket 64544
 	 */
 	public function test_render_custom_css_class_name_adds_class_to_content() {
 		$block_content = '<div class="wp-block-paragraph">Test content</div>';
@@ -239,7 +239,7 @@ public function test_render_custom_css_class_name_adds_class_to_content() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertStringContainsString( 'wp-custom-css-123abc', $result, 'Custom CSS class should be added to block content.' );
 	}
@@ -247,7 +247,7 @@ public function test_render_custom_css_class_name_adds_class_to_content() {
 	/**
 	 * Tests that render_block filter preserves existing classes when adding custom CSS class.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @ticket 64544
 	 */
 	public function test_render_custom_css_class_name_preserves_existing_classes() {
 		$block_content = '<div class="existing-class another-class">Test content</div>';
@@ -258,7 +258,7 @@ public function test_render_custom_css_class_name_preserves_existing_classes() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertStringContainsString( 'existing-class', $result, 'Existing classes should be preserved.' );
 		$this->assertStringContainsString( 'another-class', $result, 'All existing classes should be preserved.' );
@@ -268,7 +268,7 @@ public function test_render_custom_css_class_name_preserves_existing_classes() {
 	/**
 	 * Tests that render_block filter returns unchanged content when no custom CSS class in attrs.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @ticket 64544
 	 */
 	public function test_render_custom_css_class_name_returns_unchanged_when_no_custom_css_class() {
 		$block_content = '<div class="wp-block-paragraph">Test content</div>';
@@ -279,7 +279,7 @@ public function test_render_custom_css_class_name_returns_unchanged_when_no_cust
 			),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when no custom CSS class.' );
 	}
@@ -287,7 +287,7 @@ public function test_render_custom_css_class_name_returns_unchanged_when_no_cust
 	/**
 	 * Tests that render_block filter returns unchanged content when className is empty.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @ticket 64544
 	 */
 	public function test_render_custom_css_class_name_returns_unchanged_when_classname_empty() {
 		$block_content = '<div class="wp-block-paragraph">Test content</div>';
@@ -296,7 +296,7 @@ public function test_render_custom_css_class_name_returns_unchanged_when_classna
 			'attrs'     => array(),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when className is empty.' );
 	}
@@ -304,7 +304,7 @@ public function test_render_custom_css_class_name_returns_unchanged_when_classna
 	/**
 	 * Tests that render_block filter returns empty string when content is empty.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @ticket 64544
 	 */
 	public function test_render_custom_css_class_name_returns_empty_when_content_empty() {
 		$block_content = '';
@@ -315,7 +315,7 @@ public function test_render_custom_css_class_name_returns_empty_when_content_emp
 			),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertSame( '', $result, 'Result should be empty when block content is empty.' );
 	}
@@ -323,7 +323,7 @@ public function test_render_custom_css_class_name_returns_empty_when_content_emp
 	/**
 	 * Tests that custom CSS class is extracted correctly when mixed with other classes.
 	 *
-	 * @covers ::gutenberg_render_custom_css_class_name
+	 * @ticket 64544
 	 */
 	public function test_render_custom_css_class_name_extracts_class_from_mixed_classnames() {
 		$block_content = '<p>Test content</p>';
@@ -334,7 +334,7 @@ public function test_render_custom_css_class_name_extracts_class_from_mixed_clas
 			),
 		);
 
-		$result = gutenberg_render_custom_css_class_name( $block_content, $block );
+		$result = wp_render_custom_css_class_name( $block_content, $block );
 
 		$this->assertStringContainsString( 'wp-custom-css-mixed123', $result, 'Custom CSS class should be extracted and added.' );
 	}
@@ -342,7 +342,7 @@ public function test_render_custom_css_class_name_extracts_class_from_mixed_clas
 	/**
 	 * Tests that custom CSS support is enabled by default.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_support_enabled_by_default() {
 		$this->register_custom_css_block_with_support(
@@ -359,7 +359,7 @@ public function test_custom_css_support_enabled_by_default() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added by default when customCSS support is not explicitly set.' );
 	}
@@ -367,7 +367,7 @@ public function test_custom_css_support_enabled_by_default() {
 	/**
 	 * Tests that custom CSS containing HTML opening tags is rejected.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_rejects_html_opening_tags() {
 		$this->register_custom_css_block_with_support(
@@ -384,7 +384,7 @@ public function test_custom_css_rejects_html_opening_tags() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML opening tags.' );
 	}
@@ -392,7 +392,7 @@ public function test_custom_css_rejects_html_opening_tags() {
 	/**
 	 * Tests that custom CSS containing HTML closing tags is rejected.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_rejects_html_closing_tags() {
 		$this->register_custom_css_block_with_support(
@@ -409,7 +409,7 @@ public function test_custom_css_rejects_html_closing_tags() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML closing tags.' );
 	}
@@ -417,7 +417,7 @@ public function test_custom_css_rejects_html_closing_tags() {
 	/**
 	 * Tests that valid CSS without HTML markup is accepted.
 	 *
-	 * @covers ::gutenberg_render_custom_css_support_styles
+	 * @ticket 64544
 	 */
 	public function test_custom_css_accepts_valid_css() {
 		$this->register_custom_css_block_with_support(
@@ -434,7 +434,7 @@ public function test_custom_css_accepts_valid_css() {
 			),
 		);
 
-		$result = gutenberg_render_custom_css_support_styles( $parsed_block );
+		$result = wp_render_custom_css_support_styles( $parsed_block );
 
 		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added for valid CSS.' );
 	}

From f51d7693781b42d1c93ed69f504b0185dcb3328d Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Mon, 16 Feb 2026 12:59:45 +1300
Subject: [PATCH 07/12] add https://github.com/WordPress/gutenberg/pull/75052

---
 src/wp-includes/block-supports/custom-css.php | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 9a6ef1f4a34e7..9d5b13426f4ef 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -98,3 +98,36 @@ function wp_render_custom_css_class_name( $block_content, $block ) {
 add_filter( 'render_block', 'wp_render_custom_css_class_name', 10, 2 );
 add_filter( 'render_block_data', 'wp_render_custom_css_support_styles', 10, 1 );
 add_action( 'wp_enqueue_scripts', 'wp_enqueue_block_custom_css', 1 );
+
+/**
+ * Registers the style block attribute for block types that support it.
+ *
+ * @param WP_Block_Type $block_type Block Type.
+ */
+function wp_register_custom_css_support( $block_type ) {
+	// Setup attributes and styles within that if needed.
+	if ( ! $block_type->attributes ) {
+		$block_type->attributes = array();
+	}
+
+	// Check for existing style attribute definition e.g. from block.json.
+	if ( array_key_exists( 'style', $block_type->attributes ) ) {
+		return;
+	}
+
+	$has_custom_css_support = block_has_support( $block_type, array( 'customCSS' ), true );
+
+	if ( $has_custom_css_support ) {
+		$block_type->attributes['style'] = array(
+			'type' => 'object',
+		);
+	}
+}
+
+// Register the block support.
+WP_Block_Supports::get_instance()->register(
+	'custom-css',
+	array(
+		'register_attribute' => 'wp_register_custom_css_support',
+	)
+);

From 8e774afa4bd89bedc16d8c21ad9f879ba94c5c9a Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Mon, 16 Feb 2026 13:40:54 +1300
Subject: [PATCH 08/12] another test fix

---
 src/wp-includes/class-wp-theme-json.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index cf657364ebb93..b989bab4a6474 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -1526,7 +1526,7 @@ public function get_stylesheet( $types = array( 'variables', 'styles', 'presets'
 	 * @param string $selector The selector to nest.
 	 * @return string The processed CSS.
 	 */
-	public function process_blocks_custom_css( $css, $selector ) {
+	public static function process_blocks_custom_css( $css, $selector ) {
 		$processed_css = '';
 
 		if ( empty( $css ) ) {

From 6240c647da644c84aeb5fa0c9df6ec6d5c3081de Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Tue, 17 Feb 2026 11:23:34 +1300
Subject: [PATCH 09/12] Suggested updates from review

---
 src/wp-includes/block-editor.php              |   2 +-
 .../tests/block-supports/custom-css.php       | 441 ------------------
 .../wpRenderCustomCssClassName.php            | 134 ++++++
 .../wpRenderCustomCssSupportStyles.php        | 267 +++++++++++
 4 files changed, 402 insertions(+), 442 deletions(-)
 delete mode 100644 tests/phpunit/tests/block-supports/custom-css.php
 create mode 100644 tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
 create mode 100644 tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php

diff --git a/src/wp-includes/block-editor.php b/src/wp-includes/block-editor.php
index 0d9e3c0c1e067..18152756d5b73 100644
--- a/src/wp-includes/block-editor.php
+++ b/src/wp-includes/block-editor.php
@@ -658,7 +658,7 @@ function get_block_editor_settings( array $custom_settings, $block_editor_contex
 		$editor_settings = apply_filters_deprecated( 'block_editor_settings', array( $editor_settings, $post ), '5.8.0', 'block_editor_settings_all' );
 	}
 
-	$settings['canEditCSS'] = current_user_can( 'edit_css' );
+	$editor_settings['canEditCSS'] = current_user_can( 'edit_css' );
 
 	return $editor_settings;
 }
diff --git a/tests/phpunit/tests/block-supports/custom-css.php b/tests/phpunit/tests/block-supports/custom-css.php
deleted file mode 100644
index b69cc0379559c..0000000000000
--- a/tests/phpunit/tests/block-supports/custom-css.php
+++ /dev/null
@@ -1,441 +0,0 @@
-<?php
-/**
- * Test the custom CSS block support.
- *
- * @package WordPress
- */
-
-class WP_Block_Supports_Custom_CSS_Test extends WP_UnitTestCase {
-	/**
-	 * @var string|null
-	 */
-	private $test_block_name;
-
-	public function set_up() {
-		parent::set_up();
-		$this->test_block_name = null;
-	}
-
-	public function tear_down() {
-		if ( $this->test_block_name ) {
-			unregister_block_type( $this->test_block_name );
-		}
-		$this->test_block_name = null;
-		parent::tear_down();
-	}
-
-	/**
-	 * Registers a new block for testing custom CSS support.
-	 *
-	 * @param string $block_name Name for the test block.
-	 * @param array  $supports   Array defining block support configuration.
-	 *
-	 * @return WP_Block_Type The block type for the newly registered test block.
-	 */
-	private function register_custom_css_block_with_support( $block_name, $supports = array() ) {
-		$this->test_block_name = $block_name;
-		register_block_type(
-			$this->test_block_name,
-			array(
-				'api_version' => 3,
-				'attributes'  => array(
-					'style' => array(
-						'type' => 'object',
-					),
-				),
-				'supports'    => $supports,
-			)
-		);
-		$registry = WP_Block_Type_Registry::get_instance();
-
-		return $registry->get_registered( $this->test_block_name );
-	}
-
-	/**
-	 * Tests that custom CSS support adds class name when block has custom CSS.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_adds_class_name_when_css_present() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-block',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-block',
-			'attrs'     => array(
-				'style' => array(
-					'css' => 'color: red;',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added.' );
-		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should contain wp-custom-css- prefix.' );
-	}
-
-	/**
-	 * Tests that custom CSS support preserves existing className.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_preserves_existing_class_name() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-block-existing',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-block-existing',
-			'attrs'     => array(
-				'className' => 'my-existing-class',
-				'style'     => array(
-					'css' => 'color: blue;',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertStringContainsString( 'my-existing-class', $result['attrs']['className'], 'Existing className should be preserved.' );
-		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should also contain wp-custom-css- prefix.' );
-	}
-
-	/**
-	 * Tests that custom CSS support returns unchanged block when support is disabled.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_returns_unchanged_when_support_disabled() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-disabled',
-			array( 'customCSS' => false )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-disabled',
-			'attrs'     => array(
-				'style' => array(
-					'css' => 'color: green;',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when support is disabled.' );
-	}
-
-	/**
-	 * Tests that custom CSS support returns unchanged block when no CSS attribute present.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_returns_unchanged_when_no_css() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-no-css',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-no-css',
-			'attrs'     => array(
-				'style' => array(
-					'color' => 'red',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no CSS attribute.' );
-	}
-
-	/**
-	 * Tests that custom CSS support returns unchanged block when CSS is empty.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_returns_unchanged_when_css_empty() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-empty',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-empty',
-			'attrs'     => array(
-				'style' => array(
-					'css' => '',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is empty.' );
-	}
-
-	/**
-	 * Tests that custom CSS support returns unchanged block when CSS is whitespace only.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_returns_unchanged_when_css_whitespace_only() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-whitespace',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-whitespace',
-			'attrs'     => array(
-				'style' => array(
-					'css' => '   ',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS is whitespace only.' );
-	}
-
-	/**
-	 * Tests that custom CSS support returns unchanged block when style attribute is missing.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_returns_unchanged_when_no_style_attribute() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-no-style',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-no-style',
-			'attrs'     => array(),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when no style attribute.' );
-	}
-
-	/**
-	 * Tests that render_block filter adds custom CSS class to block content.
-	 *
-	 * @@ticket 64544
-	 */
-	public function test_render_custom_css_class_name_adds_class_to_content() {
-		$block_content = '<div class="wp-block-paragraph">Test content</div>';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(
-				'className' => 'wp-custom-css-123abc',
-			),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertStringContainsString( 'wp-custom-css-123abc', $result, 'Custom CSS class should be added to block content.' );
-	}
-
-	/**
-	 * Tests that render_block filter preserves existing classes when adding custom CSS class.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_render_custom_css_class_name_preserves_existing_classes() {
-		$block_content = '<div class="existing-class another-class">Test content</div>';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(
-				'className' => 'wp-custom-css-456def',
-			),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertStringContainsString( 'existing-class', $result, 'Existing classes should be preserved.' );
-		$this->assertStringContainsString( 'another-class', $result, 'All existing classes should be preserved.' );
-		$this->assertStringContainsString( 'wp-custom-css-456def', $result, 'Custom CSS class should be added.' );
-	}
-
-	/**
-	 * Tests that render_block filter returns unchanged content when no custom CSS class in attrs.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_render_custom_css_class_name_returns_unchanged_when_no_custom_css_class() {
-		$block_content = '<div class="wp-block-paragraph">Test content</div>';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(
-				'className' => 'some-other-class',
-			),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when no custom CSS class.' );
-	}
-
-	/**
-	 * Tests that render_block filter returns unchanged content when className is empty.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_render_custom_css_class_name_returns_unchanged_when_classname_empty() {
-		$block_content = '<div class="wp-block-paragraph">Test content</div>';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertSame( $block_content, $result, 'Block content should remain unchanged when className is empty.' );
-	}
-
-	/**
-	 * Tests that render_block filter returns empty string when content is empty.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_render_custom_css_class_name_returns_empty_when_content_empty() {
-		$block_content = '';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(
-				'className' => 'wp-custom-css-789ghi',
-			),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertSame( '', $result, 'Result should be empty when block content is empty.' );
-	}
-
-	/**
-	 * Tests that custom CSS class is extracted correctly when mixed with other classes.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_render_custom_css_class_name_extracts_class_from_mixed_classnames() {
-		$block_content = '<p>Test content</p>';
-		$block         = array(
-			'blockName' => 'core/paragraph',
-			'attrs'     => array(
-				'className' => 'my-class wp-custom-css-mixed123 another-class',
-			),
-		);
-
-		$result = wp_render_custom_css_class_name( $block_content, $block );
-
-		$this->assertStringContainsString( 'wp-custom-css-mixed123', $result, 'Custom CSS class should be extracted and added.' );
-	}
-
-	/**
-	 * Tests that custom CSS support is enabled by default.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_support_enabled_by_default() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-default',
-			array() // No explicit customCSS support defined.
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-default',
-			'attrs'     => array(
-				'style' => array(
-					'css' => 'font-weight: bold;',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added by default when customCSS support is not explicitly set.' );
-	}
-
-	/**
-	 * Tests that custom CSS containing HTML opening tags is rejected.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_rejects_html_opening_tags() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-html-open',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-html-open',
-			'attrs'     => array(
-				'style' => array(
-					'css' => '<script>alert(1)</script>',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML opening tags.' );
-	}
-
-	/**
-	 * Tests that custom CSS containing HTML closing tags is rejected.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_rejects_html_closing_tags() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-html-close',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-html-close',
-			'attrs'     => array(
-				'style' => array(
-					'css' => 'color: red;</style><script>alert(1)</script>',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added when CSS contains HTML closing tags.' );
-	}
-
-	/**
-	 * Tests that valid CSS without HTML markup is accepted.
-	 *
-	 * @ticket 64544
-	 */
-	public function test_custom_css_accepts_valid_css() {
-		$this->register_custom_css_block_with_support(
-			'test/custom-css-valid',
-			array( 'customCSS' => true )
-		);
-
-		$parsed_block = array(
-			'blockName' => 'test/custom-css-valid',
-			'attrs'     => array(
-				'style' => array(
-					'css' => 'color: red; background: url("image.png"); font-size: 16px;',
-				),
-			),
-		);
-
-		$result = wp_render_custom_css_support_styles( $parsed_block );
-
-		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added for valid CSS.' );
-	}
-}
diff --git a/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
new file mode 100644
index 0000000000000..5faa266ccf747
--- /dev/null
+++ b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
@@ -0,0 +1,134 @@
+<?php
+
+/**
+ * @group block-supports
+ *
+ * @covers ::wp_render_custom_css_class_name
+ */
+class Tests_Block_Supports_WpRenderCustomCssClassName extends WP_UnitTestCase {
+
+	/**
+	 * Tests that the custom CSS class name is applied to block content.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_class_name
+	 *
+	 * @dataProvider data_adds_class_to_content
+	 *
+	 * @param string $block_content  The rendered block content.
+	 * @param array  $block          The block data.
+	 * @param string $expected_class The expected class in the output.
+	 */
+	public function test_adds_class_to_content( $block_content, $block, $expected_class ) {
+		$result = wp_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertStringContainsString( $expected_class, $result, 'Custom CSS class should be present in the output.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_adds_class_to_content() {
+		return array(
+			'class is added to block content'           => array(
+				'block_content'  => '<div class="wp-block-paragraph">Test content</div>',
+				'block'          => array(
+					'blockName' => 'core/paragraph',
+					'attrs'     => array(
+						'className' => 'wp-custom-css-123abc',
+					),
+				),
+				'expected_class' => 'wp-custom-css-123abc',
+			),
+			'class is extracted from mixed class names'  => array(
+				'block_content'  => '<p>Test content</p>',
+				'block'          => array(
+					'blockName' => 'core/paragraph',
+					'attrs'     => array(
+						'className' => 'my-class wp-custom-css-mixed123 another-class',
+					),
+				),
+				'expected_class' => 'wp-custom-css-mixed123',
+			),
+		);
+	}
+
+	/**
+	 * Tests that existing classes are preserved when the custom CSS class is added.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_class_name
+	 */
+	public function test_preserves_existing_classes() {
+		$block_content = '<div class="existing-class another-class">Test content</div>';
+		$block         = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => 'wp-custom-css-456def',
+			),
+		);
+
+		$result = wp_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertStringContainsString( 'existing-class', $result, 'Existing classes should be preserved.' );
+		$this->assertStringContainsString( 'another-class', $result, 'All existing classes should be preserved.' );
+		$this->assertStringContainsString( 'wp-custom-css-456def', $result, 'Custom CSS class should be added.' );
+	}
+
+	/**
+	 * Tests that block content is returned unchanged when no custom CSS class should be applied.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_class_name
+	 *
+	 * @dataProvider data_returns_unchanged_content
+	 *
+	 * @param string $block_content The rendered block content.
+	 * @param array  $block         The block data.
+	 */
+	public function test_returns_unchanged_content( $block_content, $block ) {
+		$result = wp_render_custom_css_class_name( $block_content, $block );
+
+		$this->assertSame( $block_content, $result, 'Block content should remain unchanged.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_returns_unchanged_content() {
+		return array(
+			'no custom CSS class in attrs'  => array(
+				'block_content' => '<div class="wp-block-paragraph">Test content</div>',
+				'block'         => array(
+					'blockName' => 'core/paragraph',
+					'attrs'     => array(
+						'className' => 'some-other-class',
+					),
+				),
+			),
+			'className is not set in attrs' => array(
+				'block_content' => '<div class="wp-block-paragraph">Test content</div>',
+				'block'         => array(
+					'blockName' => 'core/paragraph',
+					'attrs'     => array(),
+				),
+			),
+			'block content is empty'        => array(
+				'block_content' => '',
+				'block'         => array(
+					'blockName' => 'core/paragraph',
+					'attrs'     => array(
+						'className' => 'wp-custom-css-789ghi',
+					),
+				),
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php b/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php
new file mode 100644
index 0000000000000..0048b379f8f33
--- /dev/null
+++ b/tests/phpunit/tests/block-supports/wpRenderCustomCssSupportStyles.php
@@ -0,0 +1,267 @@
+<?php
+
+/**
+ * @group block-supports
+ *
+ * @covers ::wp_render_custom_css_support_styles
+ */
+class Tests_Block_Supports_WpRenderCustomCssSupportStyles extends WP_UnitTestCase {
+	/**
+	 * @var string|null
+	 */
+	private $test_block_name;
+
+	public function set_up() {
+		parent::set_up();
+		$this->test_block_name = null;
+	}
+
+	public function tear_down() {
+		if ( $this->test_block_name ) {
+			unregister_block_type( $this->test_block_name );
+		}
+		$this->test_block_name = null;
+		parent::tear_down();
+	}
+
+	/**
+	 * Tests that custom CSS support adds a class name when valid CSS is present.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_support_styles
+	 *
+	 * @dataProvider data_adds_class_name
+	 *
+	 * @param string $block_name   The test block name to register.
+	 * @param array  $supports     The block support configuration.
+	 * @param array  $parsed_block The parsed block data.
+	 */
+	public function test_adds_class_name( $block_name, $supports, $parsed_block ) {
+		$this->test_block_name = $block_name;
+		register_block_type(
+			$this->test_block_name,
+			array(
+				'api_version' => 3,
+				'attributes'  => array(
+					'style' => array(
+						'type' => 'object',
+					),
+				),
+				'supports'    => $supports,
+			)
+		);
+
+		$result = wp_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayHasKey( 'className', $result['attrs'], 'Block should have className added.' );
+		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should contain wp-custom-css- prefix.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_adds_class_name() {
+		return array(
+			'class name is added when custom CSS is present'         => array(
+				'block_name'   => 'test/custom-css-block',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-block',
+					'attrs'     => array(
+						'style' => array(
+							'css' => 'color: red;',
+						),
+					),
+				),
+			),
+			'class name is added when support is not explicitly set' => array(
+				'block_name'   => 'test/custom-css-default',
+				'supports'     => array(),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-default',
+					'attrs'     => array(
+						'style' => array(
+							'css' => 'font-weight: bold;',
+						),
+					),
+				),
+			),
+			'class name is added for valid CSS with url() values'    => array(
+				'block_name'   => 'test/custom-css-valid',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-valid',
+					'attrs'     => array(
+						'style' => array(
+							'css' => 'color: red; background: url("image.png"); font-size: 16px;',
+						),
+					),
+				),
+			),
+		);
+	}
+
+	/**
+	 * Tests that existing className is preserved when custom CSS class is added.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_support_styles
+	 */
+	public function test_preserves_existing_class_name() {
+		$this->test_block_name = 'test/custom-css-block-existing';
+		register_block_type(
+			$this->test_block_name,
+			array(
+				'api_version' => 3,
+				'attributes'  => array(
+					'style' => array(
+						'type' => 'object',
+					),
+				),
+				'supports'    => array( 'customCSS' => true ),
+			)
+		);
+
+		$parsed_block = array(
+			'blockName' => 'test/custom-css-block-existing',
+			'attrs'     => array(
+				'className' => 'my-existing-class',
+				'style'     => array(
+					'css' => 'color: blue;',
+				),
+			),
+		);
+
+		$result = wp_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertStringContainsString( 'my-existing-class', $result['attrs']['className'], 'Existing className should be preserved.' );
+		$this->assertMatchesRegularExpression( '/wp-custom-css-/', $result['attrs']['className'], 'className should also contain wp-custom-css- prefix.' );
+	}
+
+	/**
+	 * Tests that custom CSS support does not add a class name when CSS should not be applied.
+	 *
+	 * @ticket 64544
+	 *
+	 * @covers ::wp_render_custom_css_support_styles
+	 *
+	 * @dataProvider data_does_not_add_class_name
+	 *
+	 * @param string $block_name   The test block name to register.
+	 * @param array  $supports     The block support configuration.
+	 * @param array  $parsed_block The parsed block data.
+	 */
+	public function test_does_not_add_class_name( $block_name, $supports, $parsed_block ) {
+		$this->test_block_name = $block_name;
+		register_block_type(
+			$this->test_block_name,
+			array(
+				'api_version' => 3,
+				'attributes'  => array(
+					'style' => array(
+						'type' => 'object',
+					),
+				),
+				'supports'    => $supports,
+			)
+		);
+
+		$result = wp_render_custom_css_support_styles( $parsed_block );
+
+		$this->assertArrayNotHasKey( 'className', $result['attrs'], 'Block should not have className added.' );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_does_not_add_class_name() {
+		return array(
+			'support is disabled'            => array(
+				'block_name'   => 'test/custom-css-disabled',
+				'supports'     => array( 'customCSS' => false ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-disabled',
+					'attrs'     => array(
+						'style' => array(
+							'css' => 'color: green;',
+						),
+					),
+				),
+			),
+			'no CSS attribute present'       => array(
+				'block_name'   => 'test/custom-css-no-css',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-no-css',
+					'attrs'     => array(
+						'style' => array(
+							'color' => 'red',
+						),
+					),
+				),
+			),
+			'CSS is empty'                   => array(
+				'block_name'   => 'test/custom-css-empty',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-empty',
+					'attrs'     => array(
+						'style' => array(
+							'css' => '',
+						),
+					),
+				),
+			),
+			'CSS is whitespace only'         => array(
+				'block_name'   => 'test/custom-css-whitespace',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-whitespace',
+					'attrs'     => array(
+						'style' => array(
+							'css' => '   ',
+						),
+					),
+				),
+			),
+			'no style attribute'             => array(
+				'block_name'   => 'test/custom-css-no-style',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-no-style',
+					'attrs'     => array(),
+				),
+			),
+			'CSS contains HTML opening tags' => array(
+				'block_name'   => 'test/custom-css-html-open',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-html-open',
+					'attrs'     => array(
+						'style' => array(
+							'css' => '<script>alert(1)</script>',
+						),
+					),
+				),
+			),
+			'CSS contains HTML closing tags' => array(
+				'block_name'   => 'test/custom-css-html-close',
+				'supports'     => array( 'customCSS' => true ),
+				'parsed_block' => array(
+					'blockName' => 'test/custom-css-html-close',
+					'attrs'     => array(
+						'style' => array(
+							'css' => 'color: red;</style><script>alert(1)</script>',
+						),
+					),
+				),
+			),
+		);
+	}
+}

From 1d72caab8e615dce21f1e662d749b91172cad780 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Tue, 17 Feb 2026 11:26:56 +1300
Subject: [PATCH 10/12] Fix linting issue

---
 .../tests/block-supports/wpRenderCustomCssClassName.php       | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
index 5faa266ccf747..0b13dfb2e2258 100644
--- a/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
+++ b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
@@ -33,7 +33,7 @@ public function test_adds_class_to_content( $block_content, $block, $expected_cl
 	 */
 	public function data_adds_class_to_content() {
 		return array(
-			'class is added to block content'           => array(
+			'class is added to block content'          => array(
 				'block_content'  => '<div class="wp-block-paragraph">Test content</div>',
 				'block'          => array(
 					'blockName' => 'core/paragraph',
@@ -43,7 +43,7 @@ public function data_adds_class_to_content() {
 				),
 				'expected_class' => 'wp-custom-css-123abc',
 			),
-			'class is extracted from mixed class names'  => array(
+			'class is extracted from mixed class names' => array(
 				'block_content'  => '<p>Test content</p>',
 				'block'          => array(
 					'blockName' => 'core/paragraph',

From f21d224d52f2bebad35a16ced5014f951fc0528d Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Tue, 17 Feb 2026 11:32:59 +1300
Subject: [PATCH 11/12] Fix linting issue

---
 .../phpunit/tests/block-supports/wpRenderCustomCssClassName.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
index 0b13dfb2e2258..0bcbc6c708468 100644
--- a/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
+++ b/tests/phpunit/tests/block-supports/wpRenderCustomCssClassName.php
@@ -33,7 +33,7 @@ public function test_adds_class_to_content( $block_content, $block, $expected_cl
 	 */
 	public function data_adds_class_to_content() {
 		return array(
-			'class is added to block content'          => array(
+			'class is added to block content'           => array(
 				'block_content'  => '<div class="wp-block-paragraph">Test content</div>',
 				'block'          => array(
 					'blockName' => 'core/paragraph',

From 0648143bfff8939dc13a96b49103f6edb6666d11 Mon Sep 17 00:00:00 2001
From: Glen Davies <glen.davies@automattic.com>
Date: Tue, 17 Feb 2026 11:50:41 +1300
Subject: [PATCH 12/12] Test fix

---
 tests/phpunit/tests/blocks/editor.php | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/phpunit/tests/blocks/editor.php b/tests/phpunit/tests/blocks/editor.php
index 4241161388eb8..1de4dbf1719a6 100644
--- a/tests/phpunit/tests/blocks/editor.php
+++ b/tests/phpunit/tests/blocks/editor.php
@@ -583,7 +583,8 @@ public function test_get_block_editor_settings_deprecated_filter_post_editor() {
 
 		$this->assertSameSets(
 			array(
-				'filter' => 'deprecated',
+				'canEditCSS' => false,
+				'filter'     => 'deprecated',
 			),
 			$settings
 		);