WordPress Themes, Plugins and XHTML Compliance

The vast majority of WordPress themes available today claim to be valid XHMTL, and indeed I have no reason to disbelieve any author who makes this statement. Problems seem to arise when you have a particular theme + a range of plugins activated and XHTML validity goes out the window. Now this may be caused by the theme, individual plugins, combinations of plugins or indeed certain theme/plugin combinations, I’m not sure. A recent example on my own site where one theme throws up 5 errors on W3C Validator and with the same plugin combination another theme throws 85 errors, whilst a 3rd has no errors. I won’t name the themes but you can see my point, as both themes claim to be valid XHTML. When a theme generates a small number of errors I will attempt to fix them but in cases where there are large numbers of errors I simply won’t bother.