I'd suggest a ::line pseudoelement for that. We already have ::first-line, which could perhaps be considered an abbreviated ::line:first-of-type. Though, ::first-line doesn't seem to support enough properties to be useful here, so maybe another approach is needed.
My only thought on this was to have a strip down the side with a vertically repeating background of an SVG data URL with the clef in it. But I don't like it. It'll be brittle. And it wouldn't support clef / key / time changes very easily.
I don’t know that this could be done cleanly without intertwining stlying and line breaking, which quite honestly sounds like a nightmare. (Line breaking is enough of a horror show as is, what with ligatures and bidi and ...)
