Re: [w3ctag/design-reviews] Element Timing API for img (#326)

Hey thanks for the review request! Some initial thoughts:

* I assume only elements that belong to the top-level browsing context are considered for auto-inclusion in the timings? E.g., many ad frameworks manipulate iframes to fill considerable space in the viewport and which oclude other content. Since iframes are not being considered in this spec (just img elements and maybe elements with background images), this is additional variability that would potentially decrease reporting value. 
* The auto-inclusion *seems* nice, but will add a tremendous amount of variability to the feature that seems like it will be challenging to rationalize and get interop across user agents--thus if an origin wants to compare data across user agents, the correlation may be difficult, leading to uncertainty and lack of confidence in the feature. 
* 'elementtiming' as an attribute on an element seems quite redundant. Perhaps something a bit shorter like 'timingname' or 'timingid'?
* Would like additional clarity on the cross-origin availability of this data. E.g., is it proposed that all "hero" timings (including the auto-added items) be available in the performance entry lists for all browsing contexts or just the top browsing context?

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