<div class="outside">
  <div class="btn">
    <span>Hover me</span>
    <div class="triangle t1"></div>
    <div class="triangle t2"></div>
  </div>
</div>
body {
  background: #4ce;
}

.outside {
  width: 500px;
  height: 100px;
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
  margin: auto;
  text-align: center;
}

.btn {
  width: 200px;
  height: 70px;
  border: 1px solid #fff;
  display: inline-block;
  position: relative;
  overflow: hidden;
}

span {
  color: #fff;
  font-size: 20px;
  line-height: 70px;
}

.triangle {
  width: 0;
  height: 0;
  position: absolute;
  left: 0;
  right: 0;
  margin: auto;
  border-left: 80px solid transparent;
  border-right: 80px solid transparent;
  opacity: .3;
  transition: all .5s;
}

.t1 {
  border-bottom: 0px solid #fff;
  bottom: 0;
}

.t2 {
  border-top: 0px solid #fff;
  top: 0;
}

.btn:hover .t1 {
  border-bottom: 120px solid #fff;
}

.btn:hover .t2 {
  border-top: 120px solid #fff;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.