/* (A) FULL SCREEN WRAPPER */
#spinner {
    position: fixed;
    top: 0; left: 0;
    width: 100vw; height: 100vh;
    z-index: 5000000000;
    background: rgba(16, 16, 16, 0.5);
    transition: opacity 0.2s;
}

/* (B) CENTER LOADING SPINNER */
#spinner img {
    position: absolute;
    top: 50%; left: 50%;
    transform: translate(-50%);
}


/* (X) DOES NOT QUITE MATTER */
html, body {
    margin: 0;
    padding: 0;
}
#loading {
    display: block;
    position: absolute;
    top: 0;
    left: 0;
    z-index: 500000000000000;
    width: 100vw;
    height: 100vh;
    background-color: rgba(16, 16, 16, 0.5);
    /*background-image: url(https://i.stack.imgur.com/FhHRx.gif);*/
    background-repeat: no-repeat;
    background-position: center;
}