button {
  padding: 15px 30px;
  border: none;
  outline: none;
  border-radius: 30px;
  font-size: 1.5rem;
  z-index: 1;
  position: relative;
  -webkit-box-shadow: 0 5px 15px rgba(33, 33, 33, .2);
  box-shadow: 0 5px 15px rgba(33, 33, 33, .2);
  overflow: hidden;
  cursor: pointer;
}

button > span {
  z-index: 2;
}

button::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  border-radius: 30px;
  background-color: #212121;
  z-index: -1;
  overflow: hidden;
  -webkit-transform: scaleX(0);
  -ms-transform: scaleX(0);
  transform: scaleX(0);
  -webkit-transform-origin: left;
  -ms-transform-origin: left;
  transform-origin: left;
  -webkit-transition: -webkit-transform 0.2s ease-in;
  transition: -webkit-transform 0.2s ease-in;
  transition: transform 0.2s ease-in;
}

button:hover {
  color: #e8e8e8;
}

button:hover::before {
  -webkit-transform: scaleX(1);
  -ms-transform: scaleX(1);
  transform: scaleX(1);
  -webkit-transform-origin: left;
  -ms-transform-origin: left;
  transform-origin: left;
}