css 边框镶角

一、background-image

效果图：

background: linear-gradient(to left, yellow, yellow) left top no-repeat,
  linear-gradient(to bottom, yellow, yellow) left top no-repeat,
  linear-gradient(to left, yellow, yellow) right top no-repeat,
  linear-gradient(to bottom, yellow, yellow) right top no-repeat,
  linear-gradient(to left, yellow, yellow) left bottom no-repeat,
  linear-gradient(to bottom, yellow,yellow) left bottom no-repeat,
  linear-gradient(to left, yellow, yellow) right bottom no-repeat,
  linear-gradient(to left, yellow, yellow) right bottom no-repeat;
  background-size: 2px 10px, 10px 2px, 2px 10px, 10px 2px;
  background-color: blue;
  border: 2px solid red;

要点：

1. linear-gradient塑造了background-image（具体渐变方向不影响结果和细节不重要，换做真实的矢量图片也可）
2. 为每一个image都指定对应的位置
3. background-size定义image的大小

也可写作：

background:linear-gradient(to left, yellow, yellow) left top / 2px 10px no-repeat,
  linear-gradient(to bottom, yellow, yellow) left top / 10px 2px no-repeat,
  linear-gradient(to left, yellow, yellow) right top  / 2px 10px no-repeat,
  linear-gradient(to bottom, yellow, yellow) right top/ 10px 2px no-repeat,
  linear-gradient(to left, yellow, yellow) left bottom  / 2px 10px no-repeat,
  linear-gradient(to bottom, yellow,yellow) left bottom / 10px 2px no-repeat,
  linear-gradient(to left, yellow, yellow) right bottom / 2px 10px no-repeat,
  linear-gradient(to left, yellow, yellow) right bottom/ 10px 2px no-repeat;
  background-color: blue;
  border: 2px solid red;

二、border-image

效果图：

border-image: url('./bg.png');
border-image-slice: 80 80 fill;
border-image-width: 40px 40px;
width: 500px;
height: 200px;

注：此处用的二倍图
要点：

1. border-image给定：边可延伸的背景图片
2. border-image-slice设置边框裁切：角不变，边延伸
   
3. border-image-width设置边框四角的宽高