Making a div that covers the entire page

I would like to make a div that covers the entire page. I put a css style with height:100% but this covers only the viewable area. I want it to also cover the area when I scroll down.

Use position:fixed this way your div will remain over the whole viewable area continuously ..

give your div a class overlay and create the following rule in your CSS

.overlay{
    opacity:0.8;
    background-color:#ccc;
    position:fixed;
    width:100%;
    height:100%;
    top:0px;
    left:0px;
    z-index:1000;
}

Demo: http://www.jsfiddle.net/TtL7R/1/

Following css will do the required job.

.overlay {
  background: #fff;
  position: fixed;
  bottom: 0;
  right: 0;
  left: 0;
  top: 0;
}

Sometimes We want to cover page body with an overlay until all resources of web page are not downloaded completely. And they we fade out this overlay to show the complete content of our web page. Following example is a little modification of above answers, In this example we show an overlay covering full body of page with css3 animation:

For more animations, visit Here

$(window).load(function() {
    $('.overlay').delay(1000).fadeOut(800);
});

.overlay {
  background: #fff;
  position: fixed;
  bottom: 0;
  right: 0;
  left: 0;
  top: 0;
}
.loading {
  position: absolute;
  top: 50%;
  left: 50%;
}
.loading-bar {
  display: inline-block;
  width: 4px;
  height: 18px;
  border-radius: 4px;
  animation: loading 1s ease-in-out infinite;
}
.loading-bar:nth-child(1) {
  background-color: #3498db;
  animation-delay: 0;
}
.loading-bar:nth-child(2) {
  background-color: #c0392b;
  animation-delay: 0.09s;
}
.loading-bar:nth-child(3) {
  background-color: #f1c40f;
  animation-delay: .18s;
}
.loading-bar:nth-child(4) {
  background-color: #27ae60;
  animation-delay: .27s;
}

@keyframes loading {
  0% {
    transform: scale(1);
  }
  20% {
    transform: scale(1, 2.2);
  }
  40% {
    transform: scale(1);
  }
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<div class="overlay">
  <div class="loading">
    <div class="loading-bar"></div>
    <div class="loading-bar"></div>
    <div class="loading-bar"></div>
    <div class="loading-bar"></div>
  </div>
</div>
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html, body { height: 100%; }
#page { min-height: 100% }

 <style type="text/css">
 html, body {
    margin:0;
    padding:0;
    height:100%; 
 } 
 #test { 
    position:absolute; 
    display:block;
    background:#ccc; 
    height:100%; 
    width:100%;
 }
 </style>

I'm not sure what you're doing with this div, but you can style the element as well.