Question

How to center a div across all browsers and behind this div there should be a transparent background layer covering entire screen of browser like lightbox.

Answer 1

If you give the div a fixed width, it’s easy to use negative margins:

div {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 600px;
    height: 400px;
    margin-top: -200px;
    margin-left: -300px;
    z-index: 20;
}

Without a fixed height, you cannot center the div vertically without JavaScript. With a dynamic height, you can vertically center the div using a snippet like this (in jQuery):

$(function() {
    var mydiv = $('div');
    mydiv.css({
        top: $(window).scrollTop() + $(window).height() / 2 - mydiv.height() / 2
    });
});

As for the transparent overlay, just give it an absolute position and a full width and height:

div#overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #000;
    opacity: 0.5;
    filter: alpha(opacity=50);
    z-index: 10;
}

If you can ditch IE6 support, you can simply use position: fixed instead of absolute, that way the divs will be centered even if the user scrolls the page, and even when JavaScript is turned off.