Maybe because 'dialog' says "don't do that" but everybody did anyway?
(also possibly because the "insert into top layer" is sufficiently odd a thing to trigger that the designers felt representing it as an attribute was a worse idea than for dialog)
... though I'd really like some sort of explanation as to how to choose between "non-modal <dialog>" and popover because the one thing I am very confident of here is that I don't know enough to answer that question myself.
As I understand the line of thought around modals there it is: If it has to be declaratively opened modal on each page visit, blocking other content, it should better be an extra page anyway. (The fact it is made possible imperatively with JS is considered "necessary evil" as I understand it.) The same presumably applies for popover; default-open popover could probably make sense in application that uses JS anyway, not a static document. (?)
Nuances of distinction between popover and non-modal dialog I have to explore.
(also possibly because the "insert into top layer" is sufficiently odd a thing to trigger that the designers felt representing it as an attribute was a worse idea than for dialog)
... though I'd really like some sort of explanation as to how to choose between "non-modal <dialog>" and popover because the one thing I am very confident of here is that I don't know enough to answer that question myself.