#container {
  text-align: center;
  margin-top: 100px;
}

#btn {
  padding: 20px 80px;
  background: #fff;
  border:1px solid #000;
  position: relative;
  overflow: hidden;
  z-index: 1;
}

#btn:before {
  content: "";
  display: inline-block;
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: #f00;
  transform: translateX(-100%);
  transition: transform 0.5s;
  z-index: -1;
}

#btn:hover::before {
  transform: translateX(0);
}