constructor(targetElem) {
    super();

    /**
     * The element that will host the looking glass.
     * @type {Element}
     * @private
     */
    this.targetElem_ = targetElem;
    this.targetElem_.setAttribute(
        'original_content', this.targetElem_.innerText);

    /**
     * The original children of the target element.
     * @type {!Array.<Node>}
     * @private
     */
    this.targetElemChildren_ = [];
  }
@param {Element} targetElem The element that will host the looking glass.