[resource hints] preconnect, preload, prender

Unfortunately won't make it to TPAC in person but seeing "prerender" topic
on the agenda would like to share some thoughts on where I'd love to see
the effort go in the coming year...

Specifically, I think the platform is missing some critical primitives that
can allow the site/app developers help the browser perform speculative
optimization: preconnects, preloads, prerenders. Yes, we do have <link
rel={dns-prefetch,prefetch,subresource,prerender}>, but in practice the
current implementations of these hints often makes them less than useful -
missing support, scheduling/prioritization gotchas, bad interop with
preloaders, inconsistent implementation between different UAs, and so on.

I think we should revisit what we've learned from our current hints and
produce a spec around the three core primitives: preconnect, preload,
prerender. To that end, drafted a quick doc describing the use cases and
motivation for each one:
https://docs.google.com/document/d/1HeTVglnZHD_mGSaID1gUZPqLAa1lXWObV-Zkx6q_HF4/edit

Would love to hear any thoughts and feedback!

P.S. Resource Hints spec as a 2014 goal? :)