[motion-1] What is the expected serialization for <position> in circle and ellipse if the explicit center is not given?

[BorisChiou]

Per some comments from https://github.com/web-platform-tests/interop/issues/340. It seems the serialization of <position> is a little bit unclear to me.

Per [css-shapes-1]

omitting components when possible without changing the meaning

And its example shows:

Omitting components means that some default values do not show up in the serialization. But since always uses the 2- or 4-value form, a default is not omitted.

So if we use circle(closest-side), what is the expected serialization? If we serialize it as circle(at 50% 50%), this seems incorrect to me because we do not use the standard default for motion path:

If circle() or ellipse() is used, and an explicit center position is not given, they default to using the offset starting position, rather than their standard default:

So I think we shouldn't serialize its default center position (for both specified value and computed value) because we don't use the standard default for circle and ellipse if the explicit center is not given. In other words, we should omit this component because it has special meaning.

cc @tabatkins @danielsakhapov

[BorisChiou]

https://github.com/w3c/csswg-drafts/issues/8695 is related to this.

[BorisChiou]

I'd like to follow the proposal mentioned in https://github.com/w3c/csswg-drafts/issues/8695#issue-1658055247:

Fix the examples and tests, and file implementation bugs, allowing at to be omitted, consistent with our general serialization principles.

[BorisChiou]

I think https://github.com/w3c/csswg-drafts/issues/8695#issuecomment-1563497665 is the proposed solution for this spec issue. (And so let's make this duplicated by https://github.com/w3c/csswg-drafts/issues/8695.)