MWTidy.php currently enables mediawiki.raggett.css conditionally for the Tidy engine only. However, the CSS is required for HTML5Depurate as well. So, the conditional enabling should be removed and @tstarling mentioned that it is best to simply merge the mediawiki.raggett.css into existing CSS files.

This has already undergone visual diff testing with HTML5Depurate which eliminates a good chunk of rendering diffs compared to Tidy.