I would also suggest checking out ABCjs for web rendering.

Not everyone needs to typeset a full symphony with all the crazy notation for every single instrument though, having an easy responsive web presentation for even 50% of the "simple" scoring you do is a fantastic option to have.

At least for Guitar, that's far from what you need even for simple scoring. I'd be surprised if you get enough support for fairly straightforward piano scores either.

Don't get me wrong, this is amazing work. It's likely enough for any kind of "chord and basic melody to help you remember enough to get through the gig". But for actual scoring, lilypond & friends are still the tool of choice. (I wish it was easier. I have spent way too much time trying to convince it to do the things I want :)

I'm hoping it'll get there, though.

But this solution won't even get you that far, let's be honest.

Can't say I share that conviction? This is absolutely perfect for stuff like "I came up with something, let me score it and share it" without having to reach for notion/musescore/etc or lilypad.

But does Lilypond generate responsive sheets?

That to me was the coolest part of this (apart from what others have said, the power of modern CSS)

They can be rendered at any size.

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.

I take that as a no.

So the measures on a line automatically wrap if viewed on a phone?

If you can make do with a more limited feature set, I'd properly go with a CSS alternative, rather than using Lilypond (depending on your environment and use case). Lilypond is fairly complicated and not without security risk and have been used as an attack vector due to it being able to embed Postscript.

LilyPond helped me to digitize my old sheet music at the time

Did you transcribe manually? Or is there some sort of service that gets most of the job done via OCR?