Why can't Node's nodeValue use [TreatNullAs=EmptyString]?

[domenic]

https://dom.spec.whatwg.org/#interface-node

The nodeValue attribute must, on setting, if the new value is null, act as if it was the empty string instead, and then do as described below, depending on the context object:

Why is this in prose instead of in IDL? Attr uses IDL for this. If there is some subtle difference, documenting it in a note would be good.

/cc @DigiTec

[ArkadiuszMichalski]

Because Node variants are nullable and: "This means that even an attribute of type DOMString? must not use [TreatNullAs], since null is a valid value of that type."
http://heycam.github.io/webidl/#TreatNullAs
#36 (comment)

[domenic]

Ah, OK, that makes sense. That seems like an arbitrary Web IDL restriction to me, but oh well. Thanks!