Should the `dialog` element be considered interactive content?

Hello!

I submitted issue validator/validator#1514 on the Nu HTML validator repository about the validity of nesting a dialog element inside of an a element. @scottaohara was very helpful and observed that this would pose an accessibility problem for people using assistive technology. Here is a quote:

i wonder if this is a miss in the HTML specification, tbh. the details element and dialog elements are both considered interactive elements, but only the details element is listed under interactive content, per your linked reference. I find that puzzling. I'd submit the validator is correct here, but rather maybe the HTML spec needs to be adjusted to indicate that putting a dialog inside of a hyperlink is invalid - as it should be.

It would seem that the spec ought to be changed so that dialog is listed under interactive content, or maybe the spec could be changed in some other way to forbid the nesting of dialog boxes in hypertext anchors.

Start by reading the HTML Standard sections for the dialog element, interactive content, and the a element, then compare their nesting rules with validator/validator#1514. Determine whether dialog should be listed as interactive content or whether a different prohibition is needed; done means the specification has a decided, normative resolution.