SMuFL: Standard Music Font Layout
smufl.org
smufl.org
Music notation is excruciatingly complex, especially when it comes to edge cases and more esoteric notations, and the artistic intents of the engravers. It's not just about "copy these notes from here to there" there's actually a ton of other aesthetic information that should be captured as well, that both makes the score more readable, and informs the performers about how to play the music.
Asking to cut & paste sections of music is basically like asking to support cut & paste for sub-fragments of SVG objects. It's "possible" but both technically difficult to implement, and of questionable utility.
To get a feel for even some "regular" cases of complex music notation take a look at the LilyPond snippet repository here: http://lsr.di.unimi.it/LSR/Browse or even at the LilyPond documentation itself: http://lilypond.org/doc/v2.19/Documentation/notation/pitches
So it's becoming a standard.
As their new year message (https://www.w3.org/community/music-notation/2015/12/10/2015-... ) says: "we look forward ... to start more extensive work on an updated web music notation format for the future."
So if you are soundly entrenched in the european art idiom of 1750-1900, this proposal must sound great, along with all the other countless standards and proposals oriented around this exceptionally narrow and historically nearly irrelevant slice of music history.
Even early choral scores, (~1200-1600) can be typeset by LilyPond using SMuFL compatible fonts in their traditional styles. (For examples, see the "Ancient Music" section of the documentation http://lilypond.org/doc/v2.18/Documentation/notation/ancient...)
Prior to that, there's not much written music, and that which is written is done so in a form so specific or esoteric that it will needs to be re-engraved into the traditional staff form for performance by modern day musicians.
There are some local variations that SMuFL doesn't really "handle" per-say, take Jianpu notation as one example. (https://en.wikipedia.org/wiki/Numbered_musical_notation) but those forms can typically be handled well by a standard text editing program.