currentColor sounds like the right default. It also seems reasonable to allow the user to override that default and set it to black or another color, depending on their use case. Maybe that's already possible? Maybe it's already possible from the command line? I haven't looked closer, but empowering users through such options is one of LilyPond's strengths.
Jan-Peter Voigt has done some work on this. He wrote on the LilyPond developer mailing list on 2018/10/16: I wrote a rudimentary engraver-based solution last year which is waiting for clean-up and completion to support MEI, MusicXML, Humdrum, LilyPond (!) and any other format for which an export-module with a defined API exists. https://github.com/jpvoigt/lilypond-export/ The code in the project is able to export a MusicXML-File for a simple lilypond-score. The resulting files are not always correct/functional...
David Garfinkle worked on MusicXML export for Google Summer of Code 2015. See the developer mailing list: https://lists.gnu.org/archive/cgi-bin/namazu.cgi?query=Garfinkle&submit=Search%21&idxname=lilypond-devel&max=20&result=normal&sort=score The code he wrote was not a full implementation/solution (complicated by Guile1.8 vs Guile2 issues), and I'm pretty sure it has not been checked in to the LilyPond repo. I'm attaching it here as a .tar file, for future reference.
Doc: document grob metadata in SVG output in Notation Reference
Pushed to staging as commit 6391910a2d123e0a4e31adffa8decc347ce59a8d Author: Paul Morris paul@paulwmorris.com Date: Thu Jun 21 00:26:20 2018 -0400 Doc: document grob metadata in SVG output in Notation Reference I.e. using the 'output-attributes' grob property to add id, class, and other attributes to the SVG output for a given grob.
Add menu http://codereview.appspot.com/357720044
We can close this issue now that issue 4561 has taken care of this quite elegantly. Not sure what the correct status(es) would be or I'd go ahead and close it myself.
Doc: CG: Add guideline for use of 'CSS' prefix in commit messages