Earlier quoted context omitted.
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 for each measure. It will wrap and re-flow, you just have to deal with the clefs separately.
And have you done that or seen it responsive?