Border Shorthand

An example of why it's sometimes important to use shorthand.

HTML

<div id="s"></div>

CSS

body {
  background-color: black;
}
#s {
  position: relative;
  overflow: hidden;
  border: 1px solid white;
  background-color: transparent;
  height: 100px;
  width: 100px;
  clip-path: polygon(25px 0, 100% 0, 100% 100%, 0 100%, 0 25px);
}
#s:before {
  content: '';
  position: absolute;
  left: -52px;
  top:25px;
  display: block;
  width: 100px;
  border-bottom: 2px solid #fff;
  transform: rotate(-45deg);
}