Method mark

lib/css/syntax.js:1997–1999

* Mark (CSS Syntax §3 "mark") — push the current cursor position. * @returns {void}

()

	 * @returns {void}
	 */
	mark() {
		this._marks.push(this._pos);
	}

	/**
	 * Restore a mark (CSS Syntax §3 "restore a mark") — pop the last mark and