"XStandard is the leading standards-compliant plug-in WYSIWYG editor for desktop applications and browser-based content management systems (IE/Mozilla/Firefox/Opera/Safari/Netscape).

The editor generates clean XHTML Strict or 1.1, uses CSS for formatting, and ensures the clean separation of content from presentation. Markup generated by XStandard meets the most demanding accessibility requirements. The editor's cool features include drag & drop file upload, spell checking and an image library that integrates tightly with your CMS.

XStandard Lite is free for commercial use."

Or maybe one can add XStandard-like custom tags and attributes to TinyMCE?