diff --git a/src/wp-includes/block-supports/custom-css.php b/src/wp-includes/block-supports/custom-css.php
index 9d5b13426f4ef..b5d977113c863 100644
--- a/src/wp-includes/block-supports/custom-css.php
+++ b/src/wp-includes/block-supports/custom-css.php
@@ -124,6 +124,153 @@ function wp_register_custom_css_support( $block_type ) {
 	}
 }
 
+/**
+ * Strips `style.css` attributes from all blocks in post content.
+ *
+ * Uses WP_Block_Parser::next_token() to scan block tokens and surgically
+ * replace only the attribute JSON that changed — no parse_blocks() +
+ * serialize_blocks() round-trip needed.
+ *
+ * @since 7.0.0
+ * @access private
+ *
+ * @param string $content Post content to filter, expected to be escaped with slashes.
+ * @return string Filtered post content with block custom CSS removed.
+ */
+function wp_strip_custom_css_from_blocks( $content ) {
+	if ( ! has_blocks( $content ) ) {
+		return $content;
+	}
+
+	$unslashed = stripslashes( $content );
+
+	$parser           = new WP_Block_Parser();
+	$parser->document = $unslashed;
+	$parser->offset   = 0;
+	$end              = strlen( $unslashed );
+	$replacements     = array();
+
+	while ( $parser->offset < $end ) {
+		$next_token = $parser->next_token();
+		list( $token_type, , $attrs, $start_offset, $token_length ) = $next_token;
+
+		if ( 'no-more-tokens' === $token_type ) {
+			break;
+		}
+
+		$parser->offset = $start_offset + $token_length;
+
+		if ( 'block-opener' !== $token_type && 'void-block' !== $token_type ) {
+			continue;
+		}
+
+		if ( ! isset( $attrs['style']['css'] ) ) {
+			continue;
+		}
+
+		// Remove css and clean up empty style.
+		unset( $attrs['style']['css'] );
+		if ( empty( $attrs['style'] ) ) {
+			unset( $attrs['style'] );
+		}
+
+		// Locate the JSON portion within the token.
+		$token_string   = substr( $unslashed, $start_offset, $token_length );
+		$json_rel_start = strcspn( $token_string, '{' );
+		$json_rel_end   = strrpos( $token_string, '}' );
+
+		$json_start  = $start_offset + $json_rel_start;
+		$json_length = $json_rel_end - $json_rel_start + 1;
+
+		// Re-encode attributes. If attrs is now empty, remove JSON and trailing space.
+		if ( empty( $attrs ) ) {
+			// Remove the trailing space after JSON.
+			$replacements[] = array( $json_start, $json_length + 1, '' );
+		} else {
+			$replacements[] = array( $json_start, $json_length, serialize_block_attributes( $attrs ) );
+		}
+	}
+
+	if ( empty( $replacements ) ) {
+		return $content;
+	}
+
+	// Build the result by splicing replacements into the original string.
+	$result = '';
+	$was_at = 0;
+
+	foreach ( $replacements as $replacement ) {
+		list( $offset, $length, $new_json ) = $replacement;
+		$result                            .= substr( $unslashed, $was_at, $offset - $was_at ) . $new_json;
+		$was_at                             = $offset + $length;
+	}
+
+	if ( $was_at < $end ) {
+		$result .= substr( $unslashed, $was_at );
+	}
+
+	return addslashes( $result );
+}
+
+/**
+ * Adds the filters to strip custom CSS from block content on save.
+ *
+ * @since 7.0.0
+ * @access private
+ */
+function wp_custom_css_kses_init_filters() {
+	add_filter( 'content_save_pre', 'wp_strip_custom_css_from_blocks', 8 );
+	add_filter( 'content_filtered_save_pre', 'wp_strip_custom_css_from_blocks', 8 );
+}
+
+/**
+ * Removes the filters that strip custom CSS from block content on save.
+ *
+ * @since 7.0.0
+ * @access private
+ */
+function wp_custom_css_remove_filters() {
+	remove_filter( 'content_save_pre', 'wp_strip_custom_css_from_blocks', 8 );
+	remove_filter( 'content_filtered_save_pre', 'wp_strip_custom_css_from_blocks', 8 );
+}
+
+/**
+ * Registers the custom CSS content filters if the user does not have the edit_css capability.
+ *
+ * @since 7.0.0
+ * @access private
+ */
+function wp_custom_css_kses_init() {
+	wp_custom_css_remove_filters();
+	if ( ! current_user_can( 'edit_css' ) ) {
+		wp_custom_css_kses_init_filters();
+	}
+}
+
+/**
+ * Initializes custom CSS content filters when imported data should be filtered.
+ *
+ * This filter is the last being executed on force_filtered_html_on_import.
+ * If the input of the filter is true it means we are in an import situation and should
+ * enable the custom CSS filters, independently of the user capabilities.
+ *
+ * @since 7.0.0
+ * @access private
+ *
+ * @param mixed $arg Input argument of the filter.
+ * @return mixed Input argument of the filter.
+ */
+function wp_custom_css_force_filtered_html_on_import_filter( $arg ) {
+	if ( $arg ) {
+		wp_custom_css_kses_init_filters();
+	}
+	return $arg;
+}
+
+add_action( 'init', 'wp_custom_css_kses_init', 20 );
+add_action( 'set_current_user', 'wp_custom_css_kses_init' );
+add_filter( 'force_filtered_html_on_import', 'wp_custom_css_force_filtered_html_on_import_filter', 999 );
+
 // Register the block support.
 WP_Block_Supports::get_instance()->register(
 	'custom-css',
diff --git a/tests/phpunit/tests/block-supports/wpStripCustomCssFromBlocks.php b/tests/phpunit/tests/block-supports/wpStripCustomCssFromBlocks.php
new file mode 100644
index 0000000000000..3d31843469642
--- /dev/null
+++ b/tests/phpunit/tests/block-supports/wpStripCustomCssFromBlocks.php
@@ -0,0 +1,129 @@
+<?php
+
+/**
+ * @group block-supports
+ *
+ * @covers ::wp_strip_custom_css_from_blocks
+ */
+class Tests_Block_Supports_WpStripCustomCssFromBlocks extends WP_UnitTestCase {
+
+	/**
+	 * Tests that style.css is stripped from block attributes.
+	 *
+	 * @ticket 64771
+	 *
+	 * @dataProvider data_strips_css_from_blocks
+	 *
+	 * @param string $content  Post content containing blocks.
+	 * @param string $message  Assertion message.
+	 */
+	public function test_strips_css_from_blocks( $content, $message ) {
+		$result = wp_unslash( wp_strip_custom_css_from_blocks( $content ) );
+		$blocks = parse_blocks( $result );
+
+		$this->assertArrayNotHasKey( 'css', $blocks[0]['attrs']['style'] ?? array(), $message );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_strips_css_from_blocks() {
+		return array(
+			'single block'                     => array(
+				'content' => '<!-- wp:paragraph {"style":{"css":"color: red;"}} --><p>Hello</p><!-- /wp:paragraph -->',
+				'message' => 'style.css should be stripped from block attributes.',
+			),
+			'empty style object is cleaned up' => array(
+				'content' => '<!-- wp:paragraph {"style":{"css":"color: red;"}} --><p>Hello</p><!-- /wp:paragraph -->',
+				'message' => 'style.css should be stripped from block attributes.',
+			),
+		);
+	}
+
+	/**
+	 * Tests that style.css is stripped from nested inner blocks.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_strips_css_from_inner_blocks() {
+		$content = '<!-- wp:group --><div class="wp-block-group"><!-- wp:paragraph {"style":{"css":"color: red;"}} --><p>Hello</p><!-- /wp:paragraph --></div><!-- /wp:group -->';
+
+		$result = wp_unslash( wp_strip_custom_css_from_blocks( $content ) );
+		$blocks = parse_blocks( $result );
+
+		$inner_block = $blocks[0]['innerBlocks'][0];
+		$this->assertArrayNotHasKey( 'css', $inner_block['attrs']['style'] ?? array(), 'style.css should be stripped from inner block attributes.' );
+	}
+
+	/**
+	 * Tests that content without blocks is returned unchanged.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_returns_non_block_content_unchanged() {
+		$content = '<p>This is plain HTML content with no blocks.</p>';
+
+		$result = wp_strip_custom_css_from_blocks( $content );
+
+		$this->assertSame( $content, $result, 'Non-block content should be returned unchanged.' );
+	}
+
+	/**
+	 * Tests that content without style.css attributes is returned unchanged.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_returns_unchanged_when_no_css_attributes() {
+		$content = '<!-- wp:paragraph {"style":{"color":{"text":"#ff0000"}}} --><p class="has-text-color" style="color:#ff0000">Hello</p><!-- /wp:paragraph -->';
+
+		$result = wp_strip_custom_css_from_blocks( $content );
+
+		$this->assertSame( $content, $result, 'Content without style.css attributes should be returned unchanged.' );
+	}
+
+	/**
+	 * Tests that other style properties are preserved when css is stripped.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_preserves_other_style_properties() {
+		$content = '<!-- wp:paragraph {"style":{"css":"color: red;","color":{"text":"#ff0000"}}} --><p>Hello</p><!-- /wp:paragraph -->';
+
+		$result = wp_unslash( wp_strip_custom_css_from_blocks( $content ) );
+		$blocks = parse_blocks( $result );
+
+		$this->assertArrayNotHasKey( 'css', $blocks[0]['attrs']['style'], 'style.css should be stripped.' );
+		$this->assertSame( '#ff0000', $blocks[0]['attrs']['style']['color']['text'], 'Other style properties should be preserved.' );
+	}
+
+	/**
+	 * Tests that empty style object is cleaned up after stripping css.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_cleans_up_empty_style_object() {
+		$content = '<!-- wp:paragraph {"style":{"css":"color: red;"}} --><p>Hello</p><!-- /wp:paragraph -->';
+
+		$result = wp_unslash( wp_strip_custom_css_from_blocks( $content ) );
+		$blocks = parse_blocks( $result );
+
+		$this->assertArrayNotHasKey( 'style', $blocks[0]['attrs'], 'Empty style object should be cleaned up after stripping css.' );
+	}
+
+	/**
+	 * Tests that slashed content is handled correctly.
+	 *
+	 * @ticket 64771
+	 */
+	public function test_handles_slashed_content() {
+		$content = '<!-- wp:paragraph {"style":{"css":"color: red;"}} --><p>Hello</p><!-- /wp:paragraph -->';
+		$slashed = wp_slash( $content );
+
+		$result = wp_strip_custom_css_from_blocks( $slashed );
+		$blocks = parse_blocks( wp_unslash( $result ) );
+
+		$this->assertArrayNotHasKey( 'css', $blocks[0]['attrs']['style'] ?? array(), 'style.css should be stripped even from slashed content.' );
+	}
+}