Actually, two questions:
How do I do this using javascript and css?
Here is the HTML, which should probably be inserted with JS, and the styles should be in an external stylesheet.
<div style="background: gray; width: 200px; height: 200px; position: absolute; left: 50%; top: 50%; margin-left: -100px; margin-top: -100px" id="modal">I'm a modal</div>
Then, you could leverage jQuery to display it.
$('a.modal').bind('click', function(event) {
event.preventDefault();
$('#modal').fadeIn(800);
});
This is only a start, you'll want to learn from this and build upon it. For example, the script should check is(':hidden')
and show, and if not then fadeOut(800)
or similiar.