未封装的js放大镜特效

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>js放大镜特效</title>
</head>
<style type="text/css">
*{
margin: 0;
padding: 0;
}
#main{
position: relative;
}
.smallimg{
300px;
height: 300px;
position: absolute;
top: 50px;
left: 50px;
}
.mark{
100px;
height:100px;
position: absolute;
z-index: 100;
top: 0;
background:rgba(0,0,0,.6);
display: none;
cursor: move;
}
.bigimg{
position: absolute;
top:50px;
left:400px;
300px;
height: 300px;
overflow: hidden;
display: none;
}
#img1{
300px;
height:300px;
display: block;
}
#img2{
900px;
height: 900px;
display: block;
}
</style>
<body>
<div id="main">
<div class="smallimg">
<img src="img/1.jpg" id="img1"/>
<div class="mark">

</div>
</div>
<div class="bigimg">
<img src="img/1.jpg" id="img2"/>
</div>
</div>
</body>
<script type="text/javascript">
window.onload=function(){

if(!document.getElementsByClassName){
document.getElementsByClassName = function(className, element){
var children = (element || document).getElementsByTagName('*');
var elements = new Array();
for (var i=0; i<children.length; i++){
var child = children[i];
var classNames = child.className.split(' ');
for (var j=0; j<classNames.length; j++){
if (classNames[j] == className){
elements.push(child);
break;
}
}
}
return elements;
};
}

//以上代码有效解决IE7、8浏览器不支持document.getElementsByClassName的问题。