.btn,
a.btn,
button.btn {
  font-size: 1.6rem;
  font-weight: 700;
  line-height: 1.5;
  position: relative;
  display: inline-block;
  padding: 1rem 4rem;
  cursor: pointer;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  -webkit-transition: all 0.3s;
  transition: all 0.3s;
  text-align: center;
  vertical-align: middle;
  text-decoration: none;
  letter-spacing: 0.1em;
  color: #fff;
  border-radius: 0.5rem;
}

a.btn-border-shadow {
  padding: calc(1.5rem - 12px) 3rem 1.5rem;

  background: #fff;
}

a.btn-border-shadow:before {
  position: absolute;
  top: -6px;
  left: -6px;

  width: 100%;
  height: 100%;

  content: "";
  -webkit-transition: all 0.3s ease;
  transition: all 0.3s ease;

  border: 1px solid #000;
  border-radius: 0.5rem;
}

a.btn-border-shadow:hover {
  padding: calc(1.5rem - 6px) 3rem;
}

a.btn-border-shadow:hover:before {
  top: 0;
  left: 0;
}

a.btn-border-shadow--color {
  border-radius: 0;
  background: #999;
}

a.btn-border-shadow--color:before {
  border-radius: 0;
}