The testing web page is here: [https://gist.github.com/wincentbalin/7a27e5b3c8485646d236d4119848953b]

Hi,
The problem comes from your user function 'img_out'.
This one is called many times, once for each SVG image, and only the first image contains the display attributes (music font).
So, just replace
    output.innerHTML = str;
by
    output.innerHTML += str;
and don't forget to clear output.innerHTML before calling abc.tosvg
(it is simpler to use a global variable, cleared before abc.tosvg(), filled in user.img_out() and copied to output.innerHTML after abc.tosvg())

Many thanks! This was the error indeed. I will update the example accordingly.

Now I tried rendering using a global variable (the script under the link above was updated accordingly).

The first time the rendering works as expected. But when pressing the Update button the second time, it looks like the styles are removed. The generated SVG code looks okay for me, but the rendered result does not. What did I misconfigure?

Sorry for the delay.
The problem comes from your abc object. Once created, it gets ABC sequences from the function tosvg() and calls user.img_out() for each SVG image. Without anymore parameter (%%fullsvg) only the first image has all the definitions (fonts/glyphs).
In your code, on update, as you use the same abc object, the SVG images have no definition after the first update.
So, the right way is to create a new abc object inside the update function.

I fixed the SVG update function in the way you described and it worked. Hence I consider this issue as resolved and this ticket as closed.

I uploaded the working testbed to the link further above in the discussion. Feel free to reference it in any documentation/discussion. The testbed is helpful for anyone who tries to program an ABC to SVG in a web page within a larger framework (wiki etc.).