I'm not referring to viewing HTML emails. I already can do that in
Claws-Mail using its Dillo plugin. I'm talking about filling in forms,
etc. that are part of the HTML email and sending just the data without
"replying" in the normal sense. This is beyond Claws' and Dillo's
capabilities. I have to use a real browser, log into that particular
web mail account (like gmail), click on that particular email, etc. to
do so.
I'm getting the sense that I may not be able to find a client that can
do that.