Mask layer only half?

html

    <div class="maak"></div>
</body>

css (body side can increase the relative effect)

body{
    position: relative;
}
.maak{
    position:absolute;

    top:0;

    bottom:0;

    left:0;

    right:0;

    background:#000;

    opacity:0.7;
    z-index: 100;
}