I just want to "inform" you: It is now nearly 100% sure that the next Internet Explorer 8 (and yes, most people still use it) is fully W3C compatible. At least CSS2.1 and XHTML.
Here is a (yet not completly finished) site: http://www.microsoft.com/windows/products/winfamily/ie/ie8/readiness/DevelopersNew.htm
I don't want to say "hey, use it", but only inform you, that you should think of validating your sites in future or - when you develop them - always have a look on the "standards" (well, they aren't yet real ISO standards, but in fact they are now). How to validate your site? http://validator.w3.org/check/referer
(CSS)What to do with the results?
- Fell happy if there is no error.
- If there are many errors: Mostly there are only quite few ones in the code itself. Just read the result carefully, always compare it to your code, and - if you don't know what to do - read at http://www.selfhtml.org/
or similar pages how to do it.