docs: function(e){
      var property = e.property;
      var ruleIds = this.get('ruleIds');
      var imageIndex = property.indexOf('background');
  
      if (imageIndex == -1) imageIndex = property.indexOf('background-image');
  
      ruleIds.push(e.id);
      if (imageIndex > -1) {
        this._collectImages(e, imageIndex);
      }
    }