Would it be possible to implement it in a static page, using just client-sige JavaScript?

You need a backend to fetch the service params and to create the invoice because of CORS.

It could be done if I setup a proxy like this which I've previously used to work with LNbits. I'd have to host the proxy separately and it wouldn't be free, so I'd prefer not to do that. Currently, I'm on the free tier on Vercel.

Thanks for replying. I see. That makes sense (CORS).

On the other hand CORS seems to me as a rather artificial barrier. Can it be still bypassed today? If not, it may be the force behind a new centralization and tracking. Not saying anything about this particular project, just thinking aloud.

You can run the app on your own machine easily following the instructions in the repo https://github.com/SamSamskies/lnurlpay

I think BOLT12 fixes this, but I doubt that's coming any time soon.

Ehm. BOLT12 works for me very well for months now.

I meant widespread adoption of it.