That's the thing, it doesn't. HTTP2 Push is one of the big "open field" of HTTP2, and to know whether a document needs to be pushed will rely on good heuristics and black magic.

There is however a small standard that's emerging, pioneered by h2o, called casper (https://h2o.examp1e.net/configure/http2_directives.html#http...). The idea is that all resources ever sent to the client are stored in a probabilistic data structure in a cookie. On every request the structure is sent back to the server, which can then check whether the resource has good chances to be already known by the browser.

By the way there are some benchmarks done by h2o's author here: http://blog.kazuhooku.com/2015/10/performance-of-http2-push-.... The conclusion is all yours.