WP_Customize_Custom_CSS_Setting::validate( string $css )

Source

File: wp-includes/customize/class-wp-customize-custom-css-setting.php

public function validate( $css ) {
		$validity = new WP_Error();

		if ( preg_match( '#</?\w+#', $css ) ) {
			$validity->add( 'illegal_markup', __( 'Markup is not allowed in CSS.' ) );
		}

		if ( ! $validity->has_errors() ) {
			$validity = parent::validate( $css );
		}
		return $validity;
	}