In my previous life I was a developer. I did a lot of work on web applications which meant I had the joy of working frequently with CSS and spent many wasted hours getting the appearance of applications just right in multiple browsers on different platforms.
IE has been heavily criticised in the past for not being compliant with web standards and there are many documented rendering bugs when it comes to CSS. Other browsers are not immune and they also have their own problems.
Today I read a post on the IE blog which was specifically on which bugs have been fixed in IE7 and also a bit of talk about future plans to become compliant. Here are the bugs they fixed:
Check out the post for more information: http://blogs.msdn.com/ie/archive/2006/08/22/712830.aspx