<a href="#" class="button" id="button">button</a>

@import url('https://fonts.googleapis.com/css?family=Rubik:500');

* {
  padding:0;
  margin:0;
}

body {
  font-family: 'Rubik', sans-serif;
  display: flex;
  -ms-flex-align: center;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  background: #CCD1D9;
}

.button {
  padding: 16px 42px;
  border-radius: 3px;
  box-shadow: 0px 0px 12px -2px rgba(0,0,0,0.5);
  line-height: 1.25;
  background: #FC6E51;
  text-decoration: none;
  user-select: none;
  color: white;
  font-size: 16px;
  letter-spacing: .08em;
  text-transform: uppercase;
  position: relative;
  transition: background-color .6s ease;
  overflow: hidden;
  &:after {
    content: "";
    position: absolute;
    width: 0;
    height: 0;
    top: 50%;
    left: 50%;
    top: var(--mouse-y);
    left: var(--mouse-x);
    transform-style: flat;
    transform: translate3d(-50%,-50%,0);
    background: rgba(white,.1);
    border-radius: 100%;
    transition: width .3s ease, height .3s ease;
  }
  &:focus,
  &:hover {
      background: darken(#FC6E51,7%);
  }
  &:active {
    &:after {
      width: 300px;
      height: 300px;
    }
  }
}

//optional js

let button = document.getElementById("button");

button.addEventListener('mousemove', (e) => {
    x = e.offsetX;
    y = e.offsetY;
    button.style.setProperty('--mouse-x', x + "px");
    button.style.setProperty('--mouse-y', y + "px");
});