-相对于父元素位置偏移
-指定偏移量：left right top bottom,负值
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<style type="text/css">
div{
width: 200px;
height: 200px;
}
#demo{
background: red;
width: 800px;
height: 800px;
margin:0px auto;
position: relative;
}
.demo1{
background: gray;
position: absolute;
left:-30px;
top: 50px;
}
.demo2{
background: green;
}
</style>
</head>
<body>
<div id="demo">
<div class="demo1">a</div>
<div class="demo2">b</div>
<div class="demo3">c</div>
</div>
</body>
</html>
