So most people use IE8 or 7 from my experience, and these web browsers don't support CSS attributes such as:
-moz-border-radius: 4px 4px 0 0;
-webkit-border-radius: 4px 4px 0 0;
border-radius: 4px 4px 0 0;
border-top-left-radius: 4px 4px;
border-top-right-radius: 4px 4px;
border-bottom-right-radius: 0px 0px;
border-bottom-left-radius: 0px 0px;
This is what controls the round border at the top of your site. This makes sure it works in firefox, chrome, ie9 etc. But this isn't in < IE8.
So the way around this would be to replace that top bar with an image. You could therefore put a div there for example, and have and image of the top bit with rounded corners as the background image.
It's very common, this is the easiest way to do it. I use it on a few of my websites, such as http://www.avnews.co.uk so every rounded corner in that is made with a background image.
Your site is not coded in an uncommon language, just something which won't work on all web browsers unfortunately. Until statistics start showing IE8 not really being used I would stay away from it.