<canvas> width/height attributes can throw?

[annevk]

When setting the value of the width or height attribute, if the context mode of the canvas element is set to placeholder, the user agent must throw an "InvalidStateError" DOMException and leave the attribute's value unchanged.

This would mean that canvas.setAttribute("width", x) can throw, which is really weird as setAttribute() cannot throw (except with Trusted Types I suppose, but that's very much opt-in and also not an accepted standard).

(The placeholder mode is a result of calling transferControlToOffscreen().)

cc @whatwg/canvas