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I think that some markup language definitely should be added, and my personal preference would be Lilypond.
I usually notate music myself, either on paper and scan it, or using Sibelius (although there are plenty of other types of software available!) I then convert to JPEG and put on the site.
Seems to work pretty well, although it would be good to input music on devices that don't have this software.
As far as I know you can input TAB directly as text.