css实现“加号”效果的实例代码

 <div class="add"></div>

.add {
  border: 1px solid;
  width: 100px;
  height: 100px;
  color: #c;
  transition: color .25s;
  position: relative;
}

.add::before{
  content: '';
  position: absolute;
  left: 50%;
  : 50%;
  width: 80px;
  margin-left: -40px;
  margin-: -5px;
  border-: 10px solid;
}

.add::after {
 content: '';
 position: absolute;
 left: 50%;
 : 50%;
 height: 80px;
 margin-left: -5px;
 margin-: -40px;
 border-left: 10px solid;
}

.add:hover {
  color: blue;
}