/* (A) LOADING SPINNER */
#spinner {
    /* (A1) COVER FULL SCREEN */
    position: fixed;
    top: 0;
    left: 0;
    z-index: 999;
    width: 100vw;
    height: 100vh;
    background: rgba(0, 0, 0, 0.7);
    transition: opacity 0.2s;
    /* (A2) CENTER SPINNER */
    display: flex;
    align-items: center;
    justify-content: center;
    /* (A3) HIDE BY DEFAULT */
    visibility: hidden;
    opacity: 0;
}

    /* (A4) SHOW SPINNER */
    #spinner.show {
        visibility: visible;
        opacity: 1;
    }

/* (B) RECOMMENDED - NO PAGE MARGIN */
html, body {
    margin: 0;
}