Can I use any HTML on an HTML permitted item or is there a subset of HTML instructions? Perhaps there is a subset on this site somewhere?

If html is allowed, there may still be other filters to remove certain potentially harmful characters or commands. For example the word 'javascript' is re-written as 'java script' by the text sanitiser.

Bad words can also be identified and re-written in the xoops administration section under
Preferences Main »» Word Censoring Options.

This option also allows admins to filter out objectionable words from any submission on the site

This Q&A was found on XOOPS Web Application System : https://xoops.org/modules/smartfaq/faq.php?faqid=389