code: function DOMChildrenWalker(el) {
      this.raw = [];
      this.index = 0;
      this.target = el;
  
      var child = el.firstChild;
      var next;
      while (child) {
          next = child.nextSibling;
  
          switch (child.nodeType) {
              case 3:
                  if (/^\s*$/.test(child.data || child.textContent)) {
                      removeEl(child);
                  }
                  else {
                      this.raw.push(child);
                  }
                  break;
  
              case 1:
              case 8:
                  this.raw.push(child);
          }
  
          child = next;
      }
  
      this.current = this.raw[this.index];
      this.next = this.raw[this.index + 1];
  }
docs: #[begin] reverse 
  元素子节点遍历操作类
  
  @inner
  @class
  @param {HTMLElement} el 要遍历的元素