This only improves performance for the initial page load by sending
resources that are almost always needed before the client receives the
preload headers and fetches them. It can degrade performance in some
edge cases such as clients with web fonts disabled or if the session
cookie is cleared without the cache being cleared. Clients can cancel
the push transfers once they start receiving them, but it's wasteful.
Safari and Firefox still support this feature but are likely to follow
the lead of Chromium and drop support for it. Few websites are going to
bother with it without Chromium support and usage is already dropping.
This avoids needing to conditionally add nopush to each preloaded
resource in the Link header. There's also no support for pushing
JavaScript modules via http2_push_preload since nginx doesn't have
support for rel=modulepreload.
