box-sizing属性

box-sizing可以分为三个值:
center-box,border和padding不计算入width之内
padding-box,padding计算入width之内
boder-box,border和padding计算入width之内，其实就是怪异模式了
使用时

-webkit-box-sizing: 100px; // for ios-safari, android
-moz-box-sizing:100px; //for ff
box-sizing:100px; //for other

栗子：

<style type="text/css">
.content-box{
box-sizing:content-box;
-moz-box-sizing:content-box;
width: 100px;
height: 100px;
padding: 20px;
border: 5px solid #E6A43F;
background: blue;
}
.padding-box{
box-sizing:padding-box;
-moz-box-sizing:padding-box;
width: 100px;
height: 100px;
padding: 20px;
border: 5px solid #186645;
background: red;
}
.border-box{
box-sizing:border-box;
-moz-box-sizing:border-box;
width: 100px;
height: 100px;
padding: 20px;
border: 5px solid #3DA3EF;
background: yellow;
}
</style>

效果图

Image.png