IE7 Dev Toolbar = Great CSS / Semantic Markup Tool
I know there are a million toolbars out there (every FireFox user will remind you so)... but I still love the simplicity and functionality of the IE7 Dev Toolbar Beta 3, and especially one of it's features that really help with CSS and semantic markup design - "Disable -> All CSS".
A "Pure CSS" design first requires good markup... so, the way you can tell if you're markup is good is by disabling all CSS styles with the dev toolbar. Here is an example of how a Pure CSS designed site will "degrade" into a proper way when you disable CSS. You can see how to do a pure CSS design from this article (How To: Pure CSS Design) which is where these images below come from. Notice how the web page below degrades.
Webpage with CSS styling:

After disabling CSS:
