Update aria-valuenow mapping to match spinbutton ARIA 1.3 changes to default value

[spectranaut]

ARIA 1.2 changes the default value of aria-valuenow on spinbutton from 0 to no default value: w3c/aria@d6a99dec

What does this mean for the aria-valuenow mapping for ATK/ATSPI and UIA, which both specify an API which returns a number? How do we represent "no value" for those APIs?

Draft WPT test: web-platform-tests/wpt#33680

[spectranaut]

For ATK, @joanmarie recommended exposing the STATE_INDETERMINATE when aria-valuenow is not present or has no value to indicate the value of 0 should not be taken seriously.

For IA2, can @asurkov and @jcsteh weigh in on what should be exposed for aria-valuenow to indicate "no default value"?

For UIA, who should we loop in?

For AXAPI, @cookiecrook, is AXValue="" ok?

[spectranaut]

Discussion from working group meeting about UIA from @smhigley: input type number has default value of zero exposed in UIA.

This might be difficult to question to answer as there is a similar "zero is the default value even when the field is empty" in html.

[cookiecrook]

Looks like the default value exposed by WebKit to the platform AX API is also 0.

The Web Accessibility Inspector doesn't display this value, but the native Accessibility Inspector does. I don't have a strong opinion for or against the change but would need to check with the platform API owners on implications of exposing an indeterminate value on steppers (the Mac name for this type of control).

Before I do, I traced impetus for this change back to @mcking65 in w3c/aria#797 where it's stated there should be an empty value allowed on stepper. I'm curious why. Considering other ranges: indeterminate progress indicators are common, but indeterminate sliders don't make sense. Are indeterminate steppers/spinbuttons a commonly used pattern? I can't immediately recall seeing one.

And in that case of a stepper/spinbutton with an indeterminate value, what value would an increment action increment to? Does it increment from unknown to… 1?

[spectranaut]

I add a new label, "blocking 1.3", because this change in ARIA happened before our new process of not merging things into ARIA until they have core-aam changes and implementation. The implementation for this has not happened in the browsers, and possibly because it is not clear how to reflect the change in the APIs.