Re: [w3ctag/design-reviews] Reference Target (Issue #961)

Addressing @LeaVerou's comment above:

> I know that, but it seems like as long as each of these ids is delegated properly, this should just work? Oh I see, you're saying you want to delegate each reference to the CE to multiple elements within its shadow root. In that case, the approach of specifying the attribute on the element seems to work even better: for attributes only taking a single idref the first one wins, for attributes taking multiple, all elements declaring a mapping are taken into account. You both seem to be saying that the order of ids is significant, but I can't find anything in the ARIA spec that says so, are we sure that's the case?

For things like `aria-labelledby` order does matter. See [Step 2B of the accessible name computation steps](https://w3c.github.io/accname/#comp_labelledby) where the IDs in the attribute are processed in order to build up the accessible name.

For example:
```html
<input type="text" aria-labelledby="b a">
<div id="a">
  two
</div>
<div id="b">
  one
</div>
```

The accessible name of the `<input>` will be "one two". So developers are accustomed to being able to control order by reordering IDs in the attribute value, without depending on DOM order.

@jyasskin said:

> @LeaVerou Check out how aria-labelledby and aria-describedby can say that an element is labeled or described by multiple other elements. It is an edge case, but it has to be supported somehow.
> 
> I'm tempted to say that it should be supported by letting those particular properties recurse, at least at the shadow root, and then you don't need anything like reference-target-map to deal with them. But I don't know enough about ARIA to be confident of that.

Changing the behavior of labelledby-describedby recursion within shadow roots seems like a recipe for confusion. There is some evidence that developers [expect and depend on these properties not chaining](https://github.com/w3c/accname/issues/209#issuecomment-1804337061). Rather than adding another element of complexity to labeledby/describedby by making them behave differently depending on context I'd prefer to just go with the version of ReferenceTarget that allows reordering these in the way that developers are already accustomed to, by ordering IDs in an attribute list.

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