It just means that there are some HTML validation problems in the pages of your site. Often, most browsers can still parse and render these ok but IE tends to not be as graceful.
The Tidy tool I mentioned above will help identify these issues so you can clean them up. I've found that if you look at a simple area of your site like the FAQ and start there, correct the issues that are often found in the header and headerinclude that all pages use, the problem will be resolved.
If you want me to take a few minutes and see if I can clean up your header/headerinclude so that it validates correctly for the vendor map just open a
ticket so I can access the AdminCP >> Styles & Templates interface and I'll take a look. It will be Monday before I can take a look though since I've got family and some work items to complete first.
Thanks!
Mark