body {
  margin: 0;
  padding: 0;

  width: 100%;
  height: 100vh;
  background-color: #f0ff7b;

  font-family: Verdana, Geneva, Tahoma, sans-serif;
}

.center {
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

.box {
  padding: 10px 15px;

  width: 300px;

  border: 1px solid rgba(0,0,0,.2);

  transition: border .5s;
}

.box::before {
  content: '';

  /* 一直居中，才在中间扩大 */
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  z-index: -1;
  
  width: 0;
  height: 0;

  background-color: #42da56;
  transition: .5s;
}

.box:hover::before {
  width: 100%;
  height: 100%;
}

.box:hover {
  color: #fff;
  transition-delay: .5s;
  border: 10px solid #fff;
}