[selectors-4] Clarify that :picture-in-picture applies to element in opener window

[SebastianZ]

The definition of the :picture-in-picture pseudo-class is vague about whether it matches the in-page (placeholder) element or the one shown within the picture-in-picture window.

Given existing implementations, it is obviously meant to match the element in the opener viewport, so it is possible to style the element as a placeholder.

Though the current spec text at least caused confusion for the implementors in Gecko.

Sebastian

[svgeesus]

@tabatkins @fantasai can this be clarified before we go to CRS, as it has caused an interop issue despite being editorial?

[svgeesus]

@SebastianZ as the intended meaning is clear, would you like to make a PR to clarify the spec?

[svgeesus]

@SebastianZ gentle nudge

[svgeesus]

Agenda+ to get a resolution on this, with input from the editors.

[tabatkins]

We don't need a resolution on this, it's just clarification. Fixed with @fantasai .

[SebastianZ]

Thank you @tabatkins and @fantasai! And sorry @svgeesus, I wasn't keeping up with my CSSWG mails lately.

Sebastian