CSS Overlay Techniques
Élément en position absolue
<html>
<body>
<div class="overlay"></div>
<body>
<html>html, body {
min-height: 100%;
}
body {
position: relative;
}
.overlay {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: 10;
background-color: rgba(0,0,0,0.5); /*dim the background*/
}