bug#33967: [Feature Request] Use proportional fonts in Info-mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 03 Jan 2019 15:35:07 -0500
> From: "Alex Griffin" <a@ajgrf.com>
> 
> Right now Emacs' built-in info viewer renders almost everything using the default face. I can set the default face to a proportional font and read most manuals just fine, but sometimes fixed-width fonts are assumed. Try using a proportional font on "(ledger3)Register Report" for example. It renders terribly in Emacs even though texinfo's html output successfully mixes proportional and fixed-width fonts [1].
> 
> It would be nice to improve it by using proportional fonts where appropriate, and fixed-width fonts only where necessary. This would improve readability and better match texinfo's html output.

AFAIU, this is not feasible as long as we use the Info format of the
manual, because 'makeinfo' formats the manual under the assumption
that it will be read using a fixed-pitch typeface, and ends each
visual line with a hard newline.  HTML format avoids that because it
leaves it to the browser to reflow and break text into lines.

AFAIK, there's work under way to improve the HTML output so that it
could provide the features we want from a GNU manual reader, and then
switch the GNU Info readers to supporting that.  But this is a
long-term goal, and cannot be reached by changes in Emacs alone.