Try this example:
var modal = document.getElementById('modal');
var shade = document.getElementById('shade');
document.getElementById('start').onclick = function() {
modal.style.display = shade.style.display = 'block';
};
document.getElementById('close').onclick = function() {
modal.style.display = shade.style.display = 'none';
};
// This code is a workaround for IE6's lack of support for the
// position: fixed style.
//
if (!('maxHeight' in document.body.style)) {
function modalsize() {
var top = document.documentElement.scrollTop;
var winsize = document.documentElement.offsetHeight;
var docsize = document.documentElement.scrollHeight;
shade.style.height = Math.max(winsize, docsize) + 'px';
modal.style.top = top + Math.floor(winsize / 3) + 'px';
};
modal.style.position = shade.style.position = 'absolute';
window.onscroll = window.onresize = modalsize;
modalsize();
}
body {
margin: 0;
}
#shade,
#modal {
display: none;
}
#shade {
position: fixed;
z-index: 100;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
#modal {
position: fixed;
z-index: 101;
top: 33%;
left: 25%;
width: 50%;
}
#shade {
background: silver;
opacity: 0.5;
filter: alpha(opacity=50);
}
<div id="shade"></div>
<div id="modal">
<textarea rows="5" cols="25"></textarea>
<button id="close">Close</button>
</div>
<p>
<button id="start">Start</button>
</p>