Improve DX of element reference attributes by allowing relative references instead of just ids

[LeaVerou]

I posted this proposal/exploration in https://github.com/whatwg/html/ 3 weeks ago but it occurred to me it would actually be an excellent fit here too.

• Improve DX of element reference attributes by allowing relative references instead of only ids whatwg/html#10143

There are several new elements also adopting this referencing scheme such as popovers, invokers, the new anchor attribute and so on, so I think it's time to address this longstanding author pain point.