On 2016-03-13 (Sun) at 20:00 (+0000), I wrote:
> I have been trying to forward HTML email (*makes the sign of the
> cross*).
>
> I want to have Gnus treat the email content "in the standard way",
> i.e. have the HTML content inline (not as an attachment), and renderable
> by non-Gnus email clients.
>
> My fiddlings with `message-forward-as-mime' and
> `message-forward-show-mml' has got me as far as having the
> HTML message inline, but the HTML is unrendered on the other end.
>
> Is there a simple way of doing this using Gnus?

I completely forgot about this thread, until just now seeing Lars's reply.

I ended up hacking together this very ugly function (more of a
script). I imagine it would have been trivial to implement this
properly, but didn't have the time to look at Gnus' internals. It works
most of the time in the way that I wanted.