The xml prologue :
It sits above the doctype and is the way that xhtml documents should be described. However IE6 has a problem with it and changes the browser from standards mode into quirks mode. The prologue is actually optional so most people leave it out anyway and is valid code either way.
In quirks mode your expressions will work (although they won't validate). Quirks mode uses the broken box model among other things so you may notice some slight differences.
Try it and see if it does what you want.
Hope that makes more sense lol
* html body (width:expression(document.body.clientWidth < 760 ? "760px" : "auto" );}