*
	 * @since 6.2.0
	 * @since 6.4.0 Changed method visibility to protected.
	 *
	 * @param string $css CSS to validate.
	 * @return true|WP_Error True if the input was validated, otherwise WP_Error.
	 */
	protected function validate_custom_css( $css ) {
		if ( preg_match( '#</?\w+#', $css ) ) {
			return new WP_Error(
				'rest_custom_css_illegal_markup',
				__( 'Markup is not allowed in CSS.' ),
				array( 'status' => 400 )
			);
		}
		return true;
	}
}