position绝对布局时，left,right,top,bottom可以同时起作用，从而撑开元素。
示例：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>position</title>
<style>
.out{
height: 200px;
position: relative;
}
.inner1{
height: 50px;
background-color: blue;
}
.inner2{
position: absolute;
top: 50px;
bottom: 0;
left: 0px;
right: 0px;
background-color: green;
}
</style>
</head>
<body>
<div class="out">
<div class="inner1"></div>
<div class="inner2"></div>
</div>
</body>
</html>
