It seems ontologically very ambiguous with respect to the "music" it is meant to represent, with each document arbitrarily positioned on a vast spectrum between plain note data that could be converted from or to MIDI events (more or less) and the opposite extreme of a printed page (e.g. CSS styles, colours, specifying rest "pitch" to position the rest symbol vertically).
In the middle of the spectrum, there are many cases of missing features (a.g. grace notes, but not trills), missing constraint (e.g. beams are only possible if the involved notes have appropriate durations), strong assumptions of traditional Western notation (but apparently no example of a standard piano part, only "groups" of independent parts), undefined behaviour (e.g. many defaults).
It's a more concise version of Music XML from the same folks. XML tends to leave rendering stuff (like drawing a page or voicing a VST instrument) to a separate style layer, so I'm not surprised to see that missing from the "MXL vs Music XML" examples linked in the story.
The project's goal was to focus on Western music theory, so seeing those "assumptions" would be expected.
I noticed that MXL tries (at first sight, with some success) to be MusicXML done better, inheriting the general style and much of the ambiguity, but not that the authors overlap.
From the article: `The existing format that comes closest to these goals is MusicXML. In fact, MNX’s creation is being led by the W3C Music Notation Community Group, which also oversees the MusicXML standard.`
In the middle of the spectrum, there are many cases of missing features (a.g. grace notes, but not trills), missing constraint (e.g. beams are only possible if the involved notes have appropriate durations), strong assumptions of traditional Western notation (but apparently no example of a standard piano part, only "groups" of independent parts), undefined behaviour (e.g. many defaults).