If it says Valid XHTML, then it MUST be valid XHTML

The content must always be validated. The first step would be to install and use https://addons.mozilla.org/firefox/249/ , an extension which uses Tidy as a simple validator. It will display an icon in the status bar which indicates the health of the document.

I've noticed inline <style> elements, which are not allowed, unquoted attributes, deprecated attributes like align, unclosed tags, and other issues on the default content and on xwiki.org.

As a general requirement: ALWAYS validate after entering some content.