demo.css

html, body {
    width: 100%;
    height: 100%;
}

body {
    border: 0;
    margin: 0;
    font-family: sans-serif;
}

a {
    color: #00f;
    font-weight: bold;
    text-decoration: none;
    cursor: pointer;
}

.demo, .popup-wrap {
    display: flex;
    align-items: center;
    justify-content: center;
}

.popup-wrap {
    position: fixed;
    top: 0;
    width: 100%;
    height: 100%;
    background:
        radial-gradient(rgba(0,0,0,0), rgba(0,0,0,0.5)),
        rgba(0,0,0,0.25);
    z-index: 1;
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.2s linear, visibility 0s;
    transition-delay: 0s, 0.2s;
}

body.popup-active .popup-wrap {
    opacity: 1;
    visibility: visible;
    transition-delay: 0s, 0s;
}

.popup {
    width: 400px;
    padding: 1em;
    text-align: center;
    background:
        rgba(255,255,255,0.9);
    border-radius: 0.5em;
    font-size: 16px;
    line-height: 1.25em;
}

.popup strong {
    font-size: 24px;
}

.demo {
    width: 100%;
    height: 100%;
    text-align: center;
    overflow: hidden;
}

@media screen and (max-width:800px) {
    .dg.ac { display:none; }
    .popup-wrap { display: none; }
}

@media screen and (min-width:801px) {
    body.popup-active > :not(.popup-wrap) {
        -webkit-filter: blur(2px);
        filter(5px);
        transform: translateZ(0);
    }
}