修改元素内容：
<body>
<p id="pid">Hello</p>
<button onclick="demo()">按钮</button>
<script>
function demo(){
var nv = document.getElementById("pid");
nv.innerHTML="World";
}
</script>
</body>修改元素标签属性：
<body>
<a href="http://www.baidu.com/" id="aid" >呵呵</a>
<button onclick="demo()">按钮</button>
<script>
function demo(){
document.getElementById("aid").href="http://www.jikexueyuan.com";
}
</script>
</body>
