writing modes and widgets' operabilities & native appearances

[Zhang-Junzhi]

I think writing modes need to be taken into account for the appearance property and widget operabilities.

Question 1:
Should we make it clear that, when the native UI controls do not support vertical writing mode, UAs must implement the according operability themselves(For example, if the operating system doesn't support vertical textfield, UAs should implement <input type="text"> in vertical writing mode themselves)?

Question 2:
What if the native UI controls of the host operating system do not support vertical writing mode when appearance is auto?
Must UAs adjust UI controls to fit the writing mode while simulating native appearances as far as possible?
Or must UAs adjust UI controls as if appearance is none.

[annevk]

I think setting the expectation that this works is good. Probably a "SHOULD" as it is UI.

But maybe we should fold this into #10039 as there's a number of properties we should clarify for native appearance.