html, body {
min-height: 100%;
}
body {
position: relative; /* needed if you position the pseudo-element absolutely */
}
body:after {
content: "";
display: block;
position: fixed; /* could also be absolute */
top: 0;
left: 0;
height: 100%;
width: 100%;
z-index: 10;
background-color: rgba(0,0,0,0.2);
}
<html>
<body>
<div class="overlay"></div>
<body>
<html>