Animation with pseudo element

animate-pseudo.scss

// html code
// <a href="#" class="btn btn--white">Button</a>

$color-grey-dark: #777;
$color-white: #fff;
$color-black: #000;

html {
  // Let 1rem = 10px
  font-size: 62.5%;
}

.btn {
  &--white {
    background-color: $color-white;
    color: $color-grey-dark;

    &::after {
      background-color: $color-white;
    }
  }

  &:link, &:visited {
    text-transform: uppercase;
    text-decoration: none;
    padding: 1.5rem 4rem;
    display: inline-block;
    border-radius: 10rem;
    transition: all .2s;
    position: relative;
    font-size: 1.6rem;
  }

  &:hover {
    transform: translateY(-.3rem);
    box-shadow: 0 1rem 2rem rgba($color-black, .2);

    &::after {
      transform: scaleX(1.4) scaleY(1.6);
      opacity: 0;
    }
  }
  
  &:active {
    transform: translateY(-.1rem);
    box-shadow: 0 .5rem 1rem rgba($color-black, .2);
  }

  &::after {
    content: "";
    display: inline-block;
    height: 100%;
    width: 100%;
    border-radius: 10rem;
    position: absolute;
    top: 0;
    left: 0;
    z-index: -1;
    transition: all .3s;
  }
}