function _startProgress() {

    //call onBeforeStart callback
    if (typeof this._onBeforeStartCallback != 'undefined') {
      this._onBeforeStartCallback.call(this);
    }

    //create the container for progress bar
    _createContainer.call(this);

    for (var i = 0, elmsLength = this._targetElement.length; i < elmsLength; i++) {
      _setProgress.call(this, this._targetElement[i]);
    }
  }
Start progress for specific element(s)

@api private
@method _createContainer