Updated aria-setsize and aria-posinset to clarify usage for authors

[MelSumner]

Resolves #747 by updating prose for aria-setsize and aria-posinset.

Still unresolved- what should UA do if one is defined and not the other?

---

Preview | Diff

[carmacleod]

Still unresolved- what should UA do if one is defined and not the other?

Good question. Looking through the spec, I see that the section on handling author errors in states and properties says:

In general, user agents do not do much validation of WAI-ARIA properties. User agents MAY do some minor validation on request, such as [...] enforcing things like aria-posinset being within 1 and aria-setsize, inclusive. User agents are not responsible for logical validation, such as the following:
...
6. Determining implicit values of aria-setsize and aria-posinset when they are specified on some but not all the elements of the set.

So whatever we decide to ask User Agents to do regarding fallback for posinset without setsize (and vice-versa) probably needs to stay within the bounds of "MAY do some minor validation". I'm not sure that UA could do much here anyhow, although perhaps...

• if posinset is specified without setsize, then UA could fallback to aria-setsize=-1 (i.e. unknown size)
• if setsize is specified without posinset, then UA could fallback to aria-posinset=1 (which would result in AT saying, for example, 1 of 50, 1 of 50, 1 of 50... which is not useful, but it would give the user an idea that "something is messed up" and they would know to ignore the reported position... ;)

[MelSumner]

@jnurthen can we add this one to the next agenda, to get complete clarification on the comments so we can ship this? Thanks!

[carmacleod]

Thanks, @MelSumner !

Giving +1 assuming the 2 suggestions about adding a conditional phrase will be committed, plus the little one-word suggestion about changing 'exposing' to 'specifying' in setsize (was already changed in posinset).

[carmacleod]

+1

Thanks @MelSumner - I think this adds clarity.

So, the only normative change is:

If authors specify aria-posinset, authors MUST also specify a value for aria-setsize.

And (correct me if I'm wrong) we didn't require the opposite (i.e. if setsize then must posinset) because we're allowing the UA's to do the posinset calculation if they know the setsize.

I won't hold up the merge for the following, but just wondering if this paragraph needs to be reworded? (Showing the posinset paragraph, but it is similar in setsize except ending in "an explicit indication of set size.")

If all items in a set are present in the document structure, it is not necessary to set this attribute , as the user agent can automatically calculate the set size and position for each item. However, if only a portion of the set is present in the document structure at a given moment, this property is needed to provide an explicit indication of an element's position.

[jnurthen]

Agree with @carmacleod
I think a small change is needed in the aria-posinset definition. Not totally happy with the wording so feel free to modify if something better comes to mind!

from

<p>If all items in a set are present in the document structure, it is not necessary to
set this <a>attribute</a>, as the <a>user agent</a> can automatically calculate the set size and position for each item. 
However, if only a portion of the set is present in the document structure at a given moment, 
this <a>property</a> is needed to provide an explicit indication of an element's position.</p>

to

<p>If all items up to the current item in a set are present in the document structure, it is not necessary to
 set this <a>attribute</a>, as the <a>user agent</a> can automatically calculate the position for these items. 
However, if all previous items in the set are not present in the document structure at a given moment, 
the author MUST set this <a>attribute</a>to provide an explicit indication of an element's position.</p>

[carmacleod]

@MelSumner I think if we just put James' suggested change into the aria-posinset definition, then this PR is ready to merge! :)