The Messages API draft and the TpMessageMixin branch implement a framework within which we can provide this feature. Capability discovery (finding out what subset of HTML we support) still needs writing.

Since capability discovery is what's missing, the next step for this bug is to research various IM protocols and describe the subsets of HTML that they support. libpurple has an enum listing various subsets, which would be a good start.

This isn't currently being worked on, but would be great to have; if anyone reading this bug wants it to happen sooner, doing the research would be a great help.

<i>This isn't currently being worked on, but would be great to have; if anyone
reading this bug wants it to happen sooner, doing the research would be a great
help.</i>

I dove into libpurple a little and I've discovered the following tags are supported:

a -- only "href" attribute
b -- converted to span style="font-weight: bold"
blockquote
body -- with "bgcolor=$BC" rewritten as [body][span style="background:$BC"]
bold -- converted to span style="font-weight: bold"
br
cite
div
em
font -- with "back=$BC", "color=$C", "face=$F", "size=$S" -- Only supports {xx-small, small, medium, large, x-large, xx-large} with values 1, 2, 3, 4, 5, 6&7 mapped respectively to those, as span style="background:$BC; color:$C font-family:$F; font-size:$S"
h1 through h6
hr -- Mapped automatically to "br"
html
i -- mapped to "em"
img -- with "src", "alt"
italic -- mapped to "em"
li
ol
p
pre
q
s -- converted to span style='text-decoration: line-through;'
script -- completely removed
span
strike -- converted to span style='text-decoration: line-through;'
strong -- converted to span style="font-weight: bold"
style -- completely removed
sub -- as span style='vertical-align:sub;'
sup -- as span style='vertical-align:super;'
td -- converted to a single tab
tr -- converted to '\n'
u -- converted to span style='text-decoration: underline;'
ul
underline -- converted to span style='text-decoration: underline;'

HTML comment is also supported.

Let me know if I can clarify anything here, or if I misunderstood what is needed.  The supported tags are most easily identified in the function libpurple/util.c:purple_markup_html_to_xhtml

(In reply to comment #3)
> I dove into libpurple a little and I've discovered the following tags are
> supported
[...]

Most of the protocols supported by libpurple support a small subset of those tags (e.g. IRC can only do bold, underline and colours; and if I remember correctly, ICQ can only change foreground/background colour for an entire message at a time, but not for e.g. individual words or sentences). We need each connection manager to be able to describe what its protocol supports, so that UIs can hide or disable controls for formatting that doesn't exist in that protocol, to avoid misleading 

To describe those capabilities, someone will need to research what levels of support exist in real-world protocols, and someone (not necessarily the same someone) will need to design a way to represent those levels of support. I believe libpurple has a system of relatively simple flags, and each protocol has some of those flags - "can do bold", "can do colours for entire messages", "can do all of XHTML-IM"? We can probably do something similar.

The HTML interface also needs to discuss security considerations, I think (whitelist-based filtering for "safe" HTML is needed).

What is the actual status of formatted text support?

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