有无box-sizing的区别
box-sizing:border-box;
有此项则该元素的元素的width包含padding和border的宽，如果无此项则元素实际大小会在width基础上再加padding和border宽度。
.div1 {
width: 300px;
height: 100px;
border: 1px solid blue;
}
.div2 {
width: 300px;
height: 100px;
padding: 50px;
border: 1px solid red;
}
.div1 {
width: 300px;
height: 100px;
border: 1px solid blue;
box-sizing: border-box;
}
.div2 {
width: 300px;
height: 100px;
padding: 50px;
border: 1px solid red;
box-sizing: border-box;
}