Only for TLS or only in beta versions. It's still going to be awhile before it's worth it to sabotage older browser performance, even once sites update their servers.

And in the end, that'll just embolden sites to crust their pages with more analytics and trackers until the performance isn't any better.

It is only for TLS, and of course your server needs to support it.

However it's definitely not just beta versions. Check it: http://caniuse.com/#search=http2

If you broaden that out to http2 and its very similar predicessor spdy then the browser support graph looks even better, including the latest versions of Safari, Mobile Safari, and IE: http://caniuse.com/#feat=spdy

>82% of US traffic supports SPDY or better.

> Only for TLS or only in beta versions.

Actually no. If you advertise in your headers that you support SPDY/HTTP2, they'll use it even if they are not using encrypted http in the first request. Anyone who hasn't updated their servers to support it can't honestly claim they care a lot about performance.

> It's still going to be awhile before it's worth it to sabotage older browser performance, even once sites update their servers.

It is already worth it really, particularly when you factor in mobile where performance is a bigger.

> And in the end, that'll just embolden sites to crust their pages with more analytics and trackers until the performance isn't any better.

Well, there is a natural equilibrium that we tend to arrive at, but at least 1st party trackers are so lightweight with SPDY as to be irrelevant. If you have one 16K image somewhere on the page, the overhead of loading 100 trackers will seem negligible (JavaScript might be another matter though ;-).