>### box-sizing box-sizing:border-box 给border,padding不会改变它的width，height box-sizing:content-box;(默认设置) 当设置padding和border时宽度会发生改变 ~~~ div{ box-sizing: border-box; border:5px solid #333; padding:10px; width:100px; height:100px; background: red; } ~~~