Same thing as in Firefox. (HTML test file included below.)
I think that mixture of fonts looks better than the Emacs one -- but the
choice is easier in a browser -- the primary font will definitely be a
proportional one, and the monospace font is secondary. And in Emacs
it's the opposite -- we can't choose a proportional font that's bigger
than the primary font, because that'll make lines with wonky heights.
OK, here's an idea: What about we don't use the default face for
monospace text in buffers with "primarily" proportional text, but use a
smaller one? Let's see...