一瞬光ってる様に見えるボタン

html

<a class="btn"><span>パステル。</span></a>

css

.btn span{
  position: relative;
  z-index: 1;
}

.btn {
  width:180px;
  display: block;
  position: relative;
  overflow: hidden;
  margin: 0 auto;
  text-align: center;
  color: #fff;
  font-size: 24px;
  padding: 17px 30px;
  background:  rgba( 145, 201, 255, 0.8);
  box-shadow: 0 3px 3px 0 rgba(0, 0, 0, 0.6);
}

.btn:before {
  content: "";
  width: 200%;
  height: 300%;
  background: rgba(255, 255, 255, 0.3);
  transform: rotate(-45deg);
  position: absolute;
  top: -20%;
  left: -280%;
  transition: 0.2s ease-in-out;
}

.btn:hover:before {
  left: 180%;
}