Clarify arrow key behavior for <input type=number> in vertical writing mode

[dizhang168]

What is the issue with the HTML Standard?

When is vertical writing-mode, it is not specced [1] what should be the arrow key behavior.
For horizontal-tb writing-mode:

• Key left should move caret position to the prev digit
• Key right should move caret position to the next digit
• Key down should step down the value (decrease)
• Key up should step up the value (increase)
• 2 arrow keys are up/down

For vertical-rl and vertical-lr, should we keep A) same behavior as horizontal
OR
B) Change to:

• Key down should move caret position to the next digit
• Key up should move caret position to the prev digit
• Key left should step down the value (decrease)
• Key right should step up the value (increase)

Further, for the rendered spin arrow buttons, should we
A) Keep 2 arrow keys as up/down
B) Change to 2 arrow keys as left/right

[1] https://drafts.csswg.org/css-writing-modes-3/#text-baselines

[domenic]

The HTML Standard does not generally specify the UI details of controls, as that is up to a user agent. For example, the fact that some user agents use spinner buttons is entirely their choice; you could also have a variety of other interfaces. (And, on different devices, you often do see other interfaces.)

So, I don't think this is really within the scope of the HTML Standard.

[annevk]

Agreed, though I can't find anything in the specification with respect to vertical rendering. How did that end up being part of Interop 2023 to begin with? @zcorpan do you know?

We also still have #4177.

[dizhang168]

I see, thank you for the explanation. Note there is also issue #8413. It would be beneficial to have some interoperability on these controls' UI, although I understand historically we have avoided to do that.

[zcorpan]

Agreed, though I can't find anything in the specification with respect to vertical rendering. How did that end up being part of Interop 2023 to begin with? @zcorpan do you know?

I'm not sure. It wasn't part of @josepharhar original test list AFAICT, and per @foolip in web-platform-tests/interop#119 (comment) only Grid and Flexbox increased the scope from 2022 to 2023, yet the .optional tests were added in web-platform-tests/wpt-metadata@bc63a7b