addStylesheetCss(serializedCss, key) {
    if (!this.document || !serializedCss) return false;
    var styleEl;
    styleEl = this._getStylesheetNode(key);
    styleEl.innerHTML = serializedCss;
    return true;
  }
Append stylesheet css
@param {string} serializedCss
@param {string} key If the key is the same, the CSS will be replaced instead of inserted