Higher level than Csound: describes the notes only, not the exact waveforms it seems.
Therefore also a bit harder to convert to actual sound: stackoverflow.com/questions/33775336/convert-musicxml-to-wav but possibly easier to convert to LilyPond.
Now they need to create a "MusicCSS" that gives the waveforms! :-)
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MusicXML is a digital sheet music interchange format designed to represent musical notation in a structured and standardized way. It allows for the easy sharing and distribution of sheet music and compositions between different software applications and systems. MusicXML files are expressed in XML (eXtensible Markup Language), which makes them both human-readable and machine-readable.