Re: [w3ctag/design-reviews] Long Animation Frame API (LoAF) (Issue #911)

Hi @noamr, thank you for proposing this idea and running it through TAG early on. Helping authors remove as much "sluggishness" as possible is always great to see.

Given the deep level of browser integration I am curious to hear your thoughts on having this interoperable across UAs. If I read the proposal correctly, you're proposing lots of lower-level details to be exposed - stages of the rendering pipeline, threading model of the browser, and additional data that might help in correlating all of this into a user action, i.e. they triggered style recalc due to `getBoundingClientRect` call etc.  
The above differs a lot between implementations and I'm concerned that micro-optimizations for one browser will tip other browsers into the opposite direction. Have you discussed this with other browser vendors?

In order to make such feature robust I am assuming you envision the ability to construct call graphs. If so, how do you envision event trigger correlation - e.g. `getBoundingClientRect` >> style load >> style parse >> style recalc etc.

Reading the S&P statements in the document and those in the template there seem to be some inconsistencies. For example, in the explainer you say 

> LoAF might expose rendering information for a particular document tree that may be cross-origin (same-agent).

while the S&P doc says

> A cross-origin iframe only receives timing information for itself.

To me this reads as a bit of contradiction, i.e. a top-level document (an iframe) can receive a cross-origin information from another document in its tree. 

Further, this information is to be exposed to all windows of the UA making it more of a global exposure. I'm probably misreading your points and if not, can you help me alleviate this concern?

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