public function add_class( $class ) {
		if ( !isset( $this->attr['class'] ) || !is_array( $this->attr['class'] ) ) {
			$this->_sanitize_class();
		}
		if ( strpos( $class, ' ' ) !== false ) {
			$class = explode( ' ', $class );
		}
		if ( is_string( $class ) ) {
			$class = array( $class );
		}
		foreach ( $class as $c ) {
			$this->attr['class'][] = $c;
		}
		return $this;
	}