Last few days I was working on fixing a lot of HTML and CSS used by the theme, and I have managed to make the theme completly XHTML 1.0 Transitional valid and also CSS level 2.1 valid. Latest public version of Starscape (1.0.2) is not valid, there are few bugs in there, but the modified code for this blog is. Next version 1.1.0 is 100% valid. But this is not always enough.

Next problem with validation is use of plugins with a theme. A lot of the widget plugins for Wordpress generate HTML that is not valid, and use CSS that is also not valid. If you try to validate CSS for this blog, you can see that NextGEN Gallery uses Thickbox that is not valid, CFormsII has small CSS problems, and Sociable also is invalid.

This problem can be solved by not using plugins that generate invalid code, and find the once that does. Or you can try to fix the problems yourself. The best solution will be for plugin authors to try and test their code as much as they can and fix such errors before publishing plugins.