transformTemplate(template, elementName) {
    if (template._gatheredStyle === undefined) {
      template._gatheredStyle = this.gatherStyles(template);
    }
    /** @type {HTMLStyleElement} */
    const style = template._gatheredStyle;
    return style ? this.transformStyle(style, elementName) : null;
  }
@param {!HTMLTemplateElement} template
@param {string} elementName
@return {StyleNode}