document.getElementsByClassName

var getElementsByClassName = function (searchClass, node,tag) {  

  if(document.getElementsByClassName){  

    var nodes =  (node || document).getElementsByClassName(searchClass),result = [];  

      for(var i=0 ;node = nodes[i++];){  

        if(tag !== "*" && node.tagName === tag.toUpperCase()){  

          result.push(node)  

        }else{  

          result.push(node)  

        }  

      }  

      return result  

    }else{  

      node = node || document;  

      tag = tag || "*";  

      var classes = searchClass.split(" "),  

      elements = (tag === "*" && node.all)? node.all : node.getElementsByTagName(tag),  

      patterns = [],  

      current,  

      match;  

      var i = classes.length;  

      while(--i >= 0){  

        patterns.push(new RegExp("(^|\\s)" + classes[i] + "(\\s|$)"));  

      }  

      var j = elements.length;  

      while(--j >= 0){  

        current = elements[j];  

        match = false;  

        for(var k=0, kl=patterns.length; k<kl; k++){  

          match = patterns[k].test(current.className);  

          if (!match)  break;  

        }  

        if (match)  result.push(current);  

      }  

      return result;  

    }  

  }