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 {

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.