phpBB uses headers h1 and h2 as built-in to organize and style forum sections and posts, so where possible, headings in posts by users should use h3, h4 and h5 instead, which also style better as content for posts.
This bbcode forum also implements the HTML tags for constructing simple tables. The implementation is intentionally simple and no more options are provided.
Because why not, we now have also added the capability for rubytext / furigana annotations. The syntax ATM is simple and limited to simple expressions, such as words; most importantly, how it will look in the end really depends on the browser you're using and the display / font configurations, but it's there for when it's wanted. It will be useful for providing annotations even in English, such as pronounciation, translation or semantic notes. Useful for writers!