html, body {
    padding: 0;
    margin:  0;
    background-color: #AAA;
}
main {
    display: flex;
    align-items: center;
    justify-content: center;
    position: absolute;
    width: 100%;
    height: 100%;
}
#viewport {
    display: flex;
    align-items: center;
    justify-content: center;
    position: relative;
    width: 100%;
    height: 100%;
    background-color: #EEE;
}