Please use the HTML base href tag to get a proper output from MODX.
I've updated the code. Unfortunately no changes in IE.
Seems to only be a problem with IE9, so I suspect it's to do with the behaviour property within the css for the rounded corners. I'm guessing whats happening is IE9 is rounding the corners with css and then the behaviour script is being applied so it's doubling up.
it's as i suspected, your HTC file isn't working on your flat page example. So it's not getting applied again in IE9.
If you view your flat pages in IE8 or IE7 no rounded corner. View the modx version and you have the rounded corners which are then double for IE9