z index

The z-index CSS property specifies the z-order of an element and its descendants. When elements overlap, z-order determines which one covers the other. An element with a larger z-index generally covers an element with a lower one.
 
For a positioned box, the z-index property specifies:
 
1) The stack level of the box in the current stacking context.
2) Whether the box establishes a local stacking context.

<!DOCTYPE html>
<html>
    <head>
        <style>
            h1 {
                position: absolute;
                left: 0px;
                top: 0px;
                z-index: -1;
            }
        </style>
    </head>
    <body>
        <h1 style="color:#FFCC99;">This is a heading</h1>
        <p>
            Because the h1 has a z-index of -1,
            it will be placed behind the text.
        </p>
    </body>
</html>