This div has height:100% because the parent div of body, html has been set to height:100%
The styles for this example are
body, html {
height:100%;/*VERY IMPORTANT because parent divs must have height of 100%*/
margin:0;/*This removes the default margin for the body*/
}
#tall {
width:980px;
height:100%;
background-color:#FFE200;
}
The KEY POINT is that parent divs must have height:100% if you want any child div to have height:100% and body, html are the top most parent divs.