Paint info?

[slightlyoff]

@yoavweiss reminded me today that we had discussed (at TPAC) the idea of including "was painted" information to the IO records.

Perhaps he can provide more information about the use-cases he had in mind.

[yoavweiss]

Apologies for not filing this earlier...

The use case I have in mind is the ability to tell when was a certain element painted on screen, in order to measure the visual impact on the user, so that it can be taken into account when talking about visual performance metrics. I'm not sure if there's a significant lag between layout and painting there, and I realize that painting info will not represent the exact time when things are actually on screen, but the more accurate the data we can have on that front, the better.

One area where things might get complicated is in scenarios where painting is done without layout. The example that comes to mind is when an image is progressively downloaded and painted, after its initial intrinsic dimensions are known and (potentially) trigger layout. Still, it would be great to tackle that scenario as well, as it would be extremely useful for visual RUM metrics.

At the same time, we need to be careful regarding privacy related attacks. In particular, I think we would need to modify the way :visited behaves in order to make it useful for scenarios where it is needed while making sure that the general attack vector (visited painting link people went to in a separate browsing session) is disabled. I'll take that up with the CSSWG.

[szager-chromium]

Anything here would definitely be a v2 feature.