docs: function(node) {
      var classesToPreserve = this._classesToPreserve;
      var className = (node.getAttribute("class") || "")
        .split(/\s+/)
        .filter(function(cls) {
          return classesToPreserve.indexOf(cls) != -1;
        })
        .join(" ");
  
      if (className) {
        node.setAttribute("class", className);
      } else {
        node.removeAttribute("class");
      }
  
      for (node = node.firstElementChild; node; node = node.nextElementSibling) {
        this._cleanClasses(node);
      }
    }