html,
body {
    margin: 0;
    padding: 0;
    height: 100%;
    width: 100%;
}

button {
    position: fixed;
    left: 50%;
    top: 50%;
    width: 200px;
    height: 100px;
    font-size: 1.6em;
    transform: translate(-50%, -50%);
}