Any tags that are not included in GTKeyboard can be added very quickly by simply requesting them. Alternately, write your own function to put into html_mode.c and send it to me.
GTKeyboard already has a long list of supported tags. You can't find a complete list here, but here's a sampling of some of the most commonly used tags in super-unorganized format:
HTML, BODY, A, B, STRONG, P, I, BR, HR, UL, LI, TABLE, TD, TR, H1, H2, H3, H4, H5, H6, PRE, comments, FONT, BLINK, etc.
There are also a few ``convenience'' buttons that include FTP and HTTP which simply print out ``ftp://''and ``http://'' respectively for quicker editing of links. There's even a ``Close Anonymous Tag'' for any closing HTML tag, that will print </> and put the cursor in the appropriate place for closing a tag off.
For those of you with special needs for HTML coding, or who want more possibilities, I suggest you check out the shortcut ability to define your own shortcuts.