A PDF is still not that responsive. Clean HTML/CSS is.
You don't have to render to PDF, you can just have some UI that keeps re-rendering Lilypond source to your screen size and zoom level every time you zoom.
Or render each measure to a separate .png file and just <img> for each measure. It will wrap and re-flow, you just have to deal with the clefs separately.
"you can just have some UI that keeps re-rendering Lilypond source to your screen size and zoom level every time you zoom."
And have you done that or seen it responsive?
If you want it to be responsive just do the Apple trick of instantly presenting a highly blurred image until the render is complete. The blurred image doesn't even have to be of the real thing, but it makes users feel warm and fuzzy.