Was looking at this problem recently, and tried:

HTML:

<body>
    <div id="my-div"></div>
</body>

CSS:

#my-div {               
    position: absolute;
    height: 100px;
    width: 100px;    
    left: 50%;
    top: 50%;
    background: red;
    transform: translate(-50%, -50%);    
    -webkit-transform: translate(-50%, -50%);
    -moz-transform: translate(-50%, -50%);
    -ms-transform: translate(-50%, -50%);
}

Here's the Fiddle:

http://jsfiddle.net/sTcp9/6/

It even works when using "width/height: auto", in the place of fixed dimensions. Tested on the latest versions on Firefox, Chrome, and IE (* gasp *).


Note: This example uses the draft version of the Flexible Box Layout Module. It has been superseded by the incompatible modern specification.

Center the child elements of a div box by using the box-align and box-pack properties together.

Example:

div
{
width:350px;
height:100px;
border:1px solid black;

/* Internet Explorer 10 */
display:-ms-flexbox;
-ms-flex-pack:center;
-ms-flex-align:center;

/* Firefox */
display:-moz-box;
-moz-box-pack:center;
-moz-box-align:center;

/* Safari, Opera, and Chrome */
display:-webkit-box;
-webkit-box-pack:center;
-webkit-box-align:center;

/* W3C */
display:box;
box-pack:center;
box-align:center;
}