Just adding to this segment as it's something I've done a bit of research for...
Firstly Ryan, you're not entirely correct, CSS does have some behavioural elements however those are pretty limited (both in support and ability), for example we can apply some behaviour effects using pseudo classes like :hover and :active (we most commonly use these type of effects with anchors, which has lead us to be able to create pure CSS drop downs, as limited as they may be). Secondly, My best estimate would put in people who have JavaScript disabled or unavailable around 8-12% of web users, when you take into account people using products like NoScript, people who actively disable scripting, those who have scripting unavailable such as some mobile devices or people with disabilities using assistive software and those using text browsers all the small niche groups tend to boost in numbers (especially in the sense of disabled visitors not being able to fully take advantage of such scripts). The best way to ensure your site isn't scripting dependant is to have a working site which doesn't use JavaScript before you start layering on the extra functionality (this is outlined as part of DOM Scripting as progressive enhancement and is the preferred method to ensuring that things work when the lights go out).
