Options for rendering musicXML "in the browser" (this is an editable page so add more if you find any!)

commercial closed source "embeddable" options:

musescore.com (one pro license for whatever you want, including from "multiple other users of your website" $60/yr)

soundslice.com "$0.50/user/month" or "$50/month for free music websites" (at least that's what they quoted me)

open source/free:

abcweb: http://wim.vree.org/js/

vexflow itself doesn't "do" musicxml but here mentions some forks of it that do/might.

related:

list on stackoverflow

alphatab doesn't yet support musicxml

http://www.musicxml.com/software/ (though hard to search through all the options and they don't seem to have much more than is listed here)

You can, of course, generate pdf's and put those (or "thumbnail images of them") on your website, as well. Or create auto-generated playback videos and put them on youtube :)