Container Definitions, manifest, and querySelectors

[mmocny]

Regarding the conversations on declarative container rules, one idea that came up in discussion is that this is basically equivalent to a style resolution and would work the same way as normal style rules (performance wise). This got me thinking... should it just be a css property?

Strawman:

body > div.mySelector {
  container-timing: 'id'; /* or just element-timing */
}

All this would do is provide a hint to the browser that it is worth buffering the timing data for elements, which would still need to be collected by the Observer.

[jdapena]

I have been thinking about this possibility... a lot. In the end we are talking about paints, so container timing is not a "declarative document model" thing only. It is the result of DOM + CSS.

Using that we have a really powerful API already available for accessing and we can still set the properties in the DOM (as style rules) or set/update the values with CSSOM. And we would already have support for adding explicit rules from chrome extensions.

The bad part is that we need to think about how it would interact with things like containment case by case. The complexity of implementing a new CSS rule is bigger usually because of that. But it would likely pay off with bigger flexibility.

We also have on the table the idea of unifying element-timing and container-timing. We could move to implement this support on top of the DOM attributes, then drop the DOM approach with a good deprecation plan (that should be easy as the audience is still not that big, and quite specialized).