.btn {
  position: relative;

  display: block;
  margin: 30px auto;
  padding: 0;

  overflow: hidden;

  border-width: 0;
  outline: none;
  border-radius: 2px;
  box-shadow: 0 1px 4px rgba(0, 0, 0, .6);
  
  background-color: #2ecc71;
  color: #ecf0f1;
  
  transition: background-color .3s;
}
.btn:disabled{
  background-color: gray;
}
.btn:not([disabled]):hover, .btn:not([disabled]):focus {
  background-color: #27ae60;
}

.btn > * {
  position: relative;
}

.btn span {
  display: block;
  padding: 12px 24px;
}

.btn:not([disabled]):before {
  content: "";
  
  position: absolute;
  top: 50%;
  left: 50%;
  
  display: block;
  width: 0;
  padding-top: 0;
    
  border-radius: 100%;
  
  background-color: rgba(236, 240, 241, .3);
  
  -webkit-transform: translate(-50%, -50%);
  -moz-transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%);
  -o-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
}

.btn:not([disabled]):active:before {
  width: 120%;
  padding-top: 120%;
  
  transition: width .2s ease-out, padding-top .2s ease-out;
}

/* Styles, not important */
*, *:before, *:after {
  box-sizing: border-box;
}