Q:

what does CSS3 and HTML5 have in common?

They are only supported in new browsers. I think everyone here knows that. But if majority of users are still on older browsers that don't support CSS3/HTML5, shouldn't that push developers to use older scripting languages (ie:HTML4/CSS2).

A:

They are both draft specifications which are partially supported (to various degrees) in current browsers.

shouldn't that push developers to use older scripting languages (ie:HTML4/CSS2).

Leaving aside neither of those being scripting languages…
Using draft features can provide a better experience in some browsers, while the fallback is still acceptable. 
JavaScript can sometimes be used to support browsers which don't have native support for a feature (although usually with a performance hit for those browsers). 
Building around draft features provides practical experience with how those features work (and helps determine if there needs to be more work on the design before the feature goes into a final specification).
Building JS based around draft features gives common ground for different developers to aim for when building scripts to achieve a specific task. This increases the interoperability and ability to switch third party scripts.

A:

No, we as developers need to push forward.  Tools like Google Chrome Frame and Explorer Canvas will allow us to push the edge without too much worry about the browsers that can't keep up.

