The CSS outline property is a confusing property. When you first learn about it, it's hard to understand how it is even remotely different from the border property. The W3C explains it as having the following differences:

1.Outlines do not take up space. 不占空间

2.Outlines may be non-rectangular. 可以不是矩形

#circle {
 width: 100px;
 height: 100px;
 background: red;
 border-radius: 50%;
 outline: solid blue;
 }