Before publishing some events from browser client I'd like to check whether certain features are available on the relays (to save some bandwidth by not spamming servers with useless requests).

When NIP-11 isn't implemented I want to assume that all capabilities are available for such relays.

Ideally I'd like to fetch relays capabilities on relays pool initialization step, without creating extra connections. Can any browser client library do that right now?

Properly doing the https GET to fetch the JSON and then upgrading to WebSocket doesn't look simple to me.

Also it requires correct server CORS configuration (currently it's not always the case).

Could we have (instead or additionally) a WebSocket request for the same JSON document?