The "Use-HTML-Wiki" mode is a user-contributed feature that is not maintained
by me.  It has a very narrow purpose of making it possible to edit wiki with
a pure HTML editor.  It should not be used for other purposes.  In my opinion,
it should not be used at all.

If you want to pure HTML document, you can check-in HTML files into the source
tree and access them as | embedded documentation].
The files are delivered to exactly as written, with no interpretation of any
kind.  And embedded documentation files also have a higher level of protection
(you have to have check-in privileges) so that they cannot be altered by
causal passers-by on the internet.  That sounds like what you want.

I'm sorry, but I cannot accept requests for different "wiki modes"
at this time.  Everybody
has their own idea of how wiki should work.  If we tried to create a new wiki
mode to suit every desire, we'd soon be smothered by dozens or hundreds of
configuration options, and we would find that the rendering worked differently
on every Fossil instance, according to the whims of the sponsor of that
instance.  I used to accept some patches for different "wiki modes".  (The
"Use HTML wiki" flag is an example and the now mostly defunct "Creole Wiki"
mode is another.) That was before I came to view "wiki modes"
as disruptive to the design integrity of the Fossil product.

Suggestions on how to improve the default wiki rendering mode, in a
backwards-compatible way, are welcomed.  I will also entertain suggestions
on how to make wiki rendering configurable so as to suit individual
preferences.  But for the later, I'm looking for a complete end-to-end
design, not a pile of hacks and patches and "mode switches" that grow
by accretion.

This ticket, if I read it correctly, is asking for more and finer-grained
mode switches that allow "Use-HTML-Wiki" to be separately turned on and
off for wiki, tickets, embedded documentation, and check-in comments.
That is not the direction we are looking to take Fossil at this time.