html, body {
    height: 100%;
}

html {
    background-color: red;
}

body {
    background-color: blue;
    margin: 0;
}

div.header {
    width: 100%;
    height: 40px;
    background-color: green;
    overflow: hidden;
}

div.content {
    height: 100%;
    height: calc(100% - 40px);
    width: 100%;
    background-color: purple;
    overflow: hidden;
}

div.cover {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 100;
    width: 100%;
    height: 100%;
    overflow: hidden;
    background-color: rgba(0, 0, 0, 0.5);
    color: #fff;
    display: none;
}

@media screen and (width: 320px) {
    html {
        height: calc(100% + 72px);
    }

    div.cover {
        display: block;
    }
}