1.

       function getByClass(classname){

                var nodes = document.getElementsByTagName('*'),

                     ret=[];

                for(var i=0;i<nodes.length;i++){

                       if(hasClass(nodes[i],classname))

                          ret.push(nodes[i]);

                }

                return ret;

            }

            function hasClass(node,className){

                var names = node.className.split(/\s+/);

                for(var i=0;i<names.length;i++){

                    if(names[i]==className)

                        return true;

                }return false;

            }

2.

<script language="javascript">

window.οnlοad=function(){

 //用className读取元数

function getClass(className,outId){

 element=document.getElementById('outId')||document;

 var temobj=element.getElementsByTagName('*')||document.all;

 classnames=[];

 for(var i=0;i<temobj.length;i++){

  var child=temobj[i];

  var classname=child.className.split(' ');

  for(var j=0;j<classname.length;j++){

   if(classname[j]==className){

    classnames.push(child);

    break;

    }

   }

  }

  return classnames;

 }

 //调用方法

 var on=getClass('tow','list');

 alert(on[0].innerHTML);

 }

</script>

3.