div相对定位

head部分

<style>
    .div1{
        width:200px;
        height:200px;
        background-color:red;
    }
    .div2{
        width:100px;
        height:100px;
        background-color:green;
        left:50px;
        top:50px;
        display:none;
        position:absolute;
    }
</style>
<script>
function show(){
    var style = document.getElementsByClassName('div2')[0].style;
    if(style.display==''||style.display=="none")
        style.display="block";
    else
        style.display="none";
}
</script>

body部分

<a href="javascript:show()">点一下</a>
<div class="div1"></div>
<div class="div2"></div>