[WICG/webcomponents] Reference Target: Which attributes are in scope? (Issue #1091)

I think we might want to be a bit more explicit about which attributes should be in scope for this feature.

l think I understand the reasoning behind wanting all attributes [1] to be in scope: I imagine it would be easier to explain that this is simply how ID/reference attributes _work_, in general. Also, adding an existing attribute into the scope of attributes which support `referenceTarget` after the feature ships would be a breaking change.

However, there are at least two attributes which seem to me to be potentially out of scope, and I don't see any WPT tests for them:

- [`headers` on `<td>`/`<tr>`](https://html.spec.whatwg.org/#attr-tdth-headers) (listed in the explainer as being in-scope)
- [`itemref`](https://html.spec.whatwg.org/#attr-itemref) - a global attribute used in Microdata (not listed in the explainer)

If we did exclude those attributes, we would need to consider how to:
1) explain to developers why some attributes don't follow `referenceTarget` forwarding
2) consider how any future attributes should opt in or out of `referenceTarget` behaviour.

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[1] The explainer states:

> This feature is intended to work with **all** attributes that refer to another element by ID string. These are:
> 
> - ARIA
>   - `aria-activedescendant`
>   - `aria-controls`
>   - `aria-describedby`
>   - `aria-details`
>   - `aria-errormessage`
>   - `aria-flowto`
>   - `aria-labelledby`
>   - `aria-owns`
> - Inputs
>   - `for` (also supports the click behavior of labels)
>   - `form`
>   - `list`
>   - `popovertarget`
>   - `anchor` (proposed in the [Popover API Explainer](https://open-ui.org/components/popover.research.explainer/#anchoring))
>   - `commandfor` (proposed in [Invokers Explainer](https://open-ui.org/components/invokers.explainer/))
>   - `interesttarget` (proposed in [Invokers Explainer](https://open-ui.org/components/invokers.explainer/))
> - Tables
>   - `headers`



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