Should all values be exposed in CSS pixels or Physical Pixels?

[mmocny]

(Context: https://crbug.com/399058544)

Should Layout Shift entry data, especially the Attribution list, be provided in CSS Pixels? In Chromium at least it seems currently to be reported in physical pixels.

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Food for thought:

• Other web platform apis and performance timeline apis (almost?) always default to CSS pixels.
• Existing spec is very vague but does refer to pixel units which are "CSS pixels" I think.
• The actual "value" of a layout shift entry is unit-less anyway, because it is a fraction of two areas, so units cancel out. Either option will yield the same result.

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I tried to read if the current spec is explicit about what is expected here.

• https://wicg.github.io/layout-instability/#sec-layout-shift-attribution
• https://wicg.github.io/layout-instability/#create-the-attribution
• https://wicg.github.io/layout-instability/#sec-basic-concepts

And the only explicit reference I see is to "pixel units" (https://www.w3.org/TR/css-values-4) which my reading suggests should be "CSS Pixels" -- so perhaps the spec already intends for this behaviour.

[mmocny]

Discussed at Web Perf WG today, and sentiment in the room was to go ahead and make the change.

The existing users of the api (in the room, at least) were mostly using it just to report information about the node with the largest shift, rather than the specific details of the area shifted.

However, there are a few tools that try to visualize shifted areas, i.e. into an animated video, and these are more likely to rely on these values. Will reach out to them.

[anediaz]

Same here, I couldn't find any explicit statement in the official documentation or specs confirming that DOMRectReadOnly values are in CSS pixels. It seems to be an implicit standard that all DOM geometry is defined in CSS pixels.

Also, I've reviewed all the Web Performance APIs and haven't found any others that report values in physical pixels. While ResizeObserver does expose devicePixelContentBoxSize in physical pixels, it's not a dedicated performance API and can be excluded.