[Beacon] spec feedback + few suggestions

Google search is rolling out <a ping> to mobile clients that support it and
based on our measurements its saving 200-400ms on every click [1]. The
latency savings by themselves are, of course, a huge win, but just as
importantly, our metrics also show a significant improvement in number of
successful navigations (one less redirect to go wrong + faster load of
destination site).

a) Above should address the previously raised concerns over usefulness of
<a ping> (and by implication, Beacon).
b) <a ping> lacks cross-browser support (limited to Safari and Chrome),
which makes deployment a non-trivial exercise.

As a result, I believe that having a cross-browser implementation of Beacon
API would be a big performance win. Plus, Beacon is also much more flexible
and would enable many additional optimization opportunities, including
optimizing for battery life of mobile handsets... In short, we need Beacon
API!

On that note, Jonas and myself have put together a doc outlining some
feedback and suggestions on current proposal:
https://docs.google.com/document/d/1KSDg9ji53l-IrL6U5g1vvD96KIbZoQj1iUX4KwVvORM/edit#

Would love to hear any thoughts or feedback.

ig

[1] https://plus.google.com/+IlyaGrigorik/posts/fPJNzUf76Nx