Question

<div id="wr">
    <div id="unknownWidthAndHeight">should be centered on both sides</div>
</div>

#wr {
    display:table-cell;
    width:400px;
    height:100px;
    border:1px solid red;
    margin:50px;
    vertical-align:middle;
}
#unknownWidthAndHeight{
    display:table;
    height:30px;
    margin:auto;    
    border:1px solid blue;
}

Here is fiddle example:
http://jsfiddle.net/gdTGZ/2/

Need such support for IE7 without display:table etc. and without <table> usage.

Answer 1

1/ If you want to vertical center on IE7 try this technique using three divs :
http://www.jakpsatweb.cz/css/css-vertical-center-solution.html

Ps : You’ll always need to set the container height.

2/ The element that you want to horizontal center must have a width and margin:0 auto; otherwise you can try text-align:center