Currently it appears to not be, in three ways:
- It gets cleared (set to undefined) when an iframe is removed from the DOM
- It gets reset to a new value on navigation from the initial about:blank, even though the Window stays the same during such a navigation
- It does not get cleared during document.open(). Per spec this is supposed to create a new Window; non-Firefox browsers do not follow this, but they do generally clear Window-associated stuff, and so we should do the same for window.customElements.
Tests at http://w3c-test.org/custom-elements/custom-element-registry/per-global.html
This stems from the fact neither Blink nor WebKit preserves DOMWindow when navigating from about:blank and doesn't replace DOMWindow upon document.open.
This seems quite inconsequential nonetheless.
We will likely revert the document.open() tests as part of a general document.open() overhaul led by Timothy Gu (CC'ed), to bring the spec more in line with the simple model used by Chrome and WebKit.
For about:blank, the issue is less clear to me, as I was pretty sure that did preserve Window in all browsers... I'm investigating further and will get back to you.
Great. Let me know what you decide to do.