Update "undefined" definition in section 6.2.4 Value (within 6.2 Characteristics of States and Properties)

[rahimabdi]

Closes https://github.com/w3c/aria/issues/2156 Closes https://github.com/w3c/aria/issues/2177

This PR clarifies ambiguity in the varying behaviors of the undefined value for ARIA states and properties by providing a broader definition of "undefined" in section 6.2.4 Value.

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[rahimabdi]

Awaiting resolution on https://github.com/w3c/aria/issues/2177 before updating PR with new undefined definition.

For future work on this PR (and to address feedback above):

• The definition should probably avoid referencing specific properties/states (or reference just aria-expanded as @smhigley notes)
• Should include link to IDL reflection section
• Should clarify how undefined is assigned (it sounds like it should be as the value, not the string)

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[rahimabdi]

Almost there!

@smhigley @aardrian you mentioned adding a link to the IDL reflection section (section 10. IDL Interface) although I'm not seeing any immediate relation to undefined assignment. Section 6.3 ARIA Attributes does provide examples of IDL assignment but please let me know which link you prefer I include. Thank you!

[smockle]

@rahimabdi Does this PR need to be updated in light of the July 25th ARIA WG meeting discussion?