When using FF,it has a webdeveloper bar as add on ( http://chrispederick.com/work/web-developer/ )
It shows directly the validation in this bar.
The css must be validated and the html, so it's correct to show in all mainstream browsers.
But javascript will never pass because variables you cannot validate
But you can learn from the warnings and adjust them.
The warnings usually show recommendations.
If you got errors,now there is something wrong and it might not work.
(FYI:some programs or scripters allready use new CSS-3 features that are not going to be validated in CSS-2)

Hans