Re: [w3ctag/design-reviews] Paint/presentation timestamps in performance APIs (Issue #1013)

noamr left a comment (w3ctag/design-reviews#1013)

> Hi [@noamr](https://github.com/noamr), the TAG discussed it today and found some new concerns:
> 
> We are aware that the "presentation time" is not well defined, and an accurate definition might be out of the scope of the Web specs. However, the "paint time" as how it's currently defined is not perceivable by the end user. It's somewhere in the middle of the entire rendering pipeline, and there might be an arbitrary delay between this and when the pixel actually shows up on the screen. For this reason, we are unsure whether this "paint time" is a good enough performance indicator to add to the platform.

It is the latest interoperable timestamp. It gives the developer indications about what they did that could have contributed to a long paint. It is also consistent across different performance entry types (e.g. LCP and LoAF), which allows correlation.
It's a very useful timestamp.

Note that most timestamps in existing entries (e.g. resource timing) are not directly reflecting UX, but are used for *attribution* - finding the root cause for problematic user experience. Paint timing is one of those.

> 
> We are also wondering if new timings in the rendering pipeline could be used to perform side-channel attacks, e.g. exfiltrate canvas data to someone else.

Presentation times are heavily coarsened when not in a cross-origin isolated environment for that reason.

> 
> Have you considered alternatives that are more interoperable and better match the page performance from the end users' perspective?

This has been discussed for about 4 years, see https://github.com/w3c/paint-timing/issues/62.
So, yes, we have considered this, and this is the best alternative we could come up with.

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