[WICG/webcomponents] Reference Target: what type should the `referenceTarget` attribute be? (Issue #1093)

> @alice To make sure I understand you, are you thinking that the `referenceTarget` attribute's type should be a nullable `DOMString`?
> 
> ```js
> partial interface ShadowRoot {
>   attribute DOMString? referenceTarget;
> }
> ```
> 
> I had been assuming the attribute's type would just be `DOMString`, and that's how it's [prototyped today](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/dom/shadow_root.idl;l=58;drc=86547d0ef65244867832bba1721ef3d0ed826eea). But looking at the explainer again I guess it's not specific on this point.
> 
> Assuming I'm reading the specs correctly, keeping it as a non-nullable `DOMString` triggers [this string conversion](https://webidl.spec.whatwg.org/#js-DOMString) when trying to set the attribute to null, and this conversion happens before the implementation of the property "sees" the value:
> ```js
> document.querySelector("#fancy-listbox").shadowRoot.referenceTarget = null;
> document.querySelector("#fancy-listbox").shadowRoot.referenceTarget; // logs 'null'
> ```
> But if we make it `DOMString?` then we'd get the behavior you suggest. That's definitely seems like it'd avoid a pitfall when someone wants to clear the referenceTarget value. An inconsistency it introduces though is that the `Element.id` property is itself a non-nullable `DOMString`. So you get this today:
> 
> ```js
> const div = document.createElement("div");
> div.id = null;
> div.id; // logs 'null'
> ```
> 
> It's tempting to treat the `referenceTarget` property as kind of equivalent to setting an ID elsewhere, so keeping the behavior consistent could be nice. What do you think? (Maybe this question should be its own thread.) 

 _Originally posted by @dandclark in [#1089](https://github.com/WICG/webcomponents/issues/1089#issuecomment-2606132790)_

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