Edit in JSFiddle

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@keyframes bouncing{
  0%  { bottom: 0; box-shadow: 0 0 5px rgba(0,0,0,0.5);}
  100%{ bottom: 50px; box-shadow: 0 50px 50px rgba(0,0,0,0.1);}
}

a {
    animation: bouncing 0.5s cubic-bezier(.1,.25,.1,1) 0s infinite alternate both;
    background: lightslategrey;
    border-radius: 2px;
    display: inline-block;
    color: white;
    cursor: pointer;
    font-size: 0.75rem;
    font-weight: 300;
    letter-spacing: 0.2em;
    padding: 1em 2em 1.1em;
    position: relative;
    text-decoration: none;
    text-transform: uppercase;
    vertical-align: top;
    margin-top: 50px;
}