Default keepalive time of two hours is also quite long, in some situations, where you would expect to get the notification just a bit faster.

Yes, default values are not sane for most connections.

Good luck access this from browser.

Browsers can do this.

Aha, show me how i can access low level TCP stuff from JS. Browsers could do this, but they would not.

There is no need for that in JS. Browser (and any other program) can open connections with SO_KEEPALIVE.

I bet they could not. It will ruin whole internet with little gain since you still need application level pings anyway because issue could be in a broken server, not on TCP level.

i bet you missed the times when uTorrent turned on uTP and almost everywhere in the world ISP hardware just failed because of x100 traffic.