" _updateRendering() { // Left as an exerci..."

[o-t-w]

https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-autonomous-example

_updateRendering() {
// Left as an exercise for the reader. But, you'll probably want to
// check this.ownerDocument.defaultView to see if we've been
// inserted into a document with a browsing context, and avoid
// doing any work if not.
}

I don't think a spec is the best place for a pop-quiz. It would be nice to see a fleshed out example for people trying to get there heads around custom elements.

[domenic]

Well, the point of this example isn't to show how to render a country flag. It's to show how custom elements work. So I'd rather not distract from the point by spending a bunch of space on country flag rendering.

[o-t-w]

Yes, that's fair enough. Would you be kind enough to educate me on what a document without a browsing context would be? I know this is meant as a spec for implementers and not a tutorial...but there aren't many decent resources on vanilla custom elements right now so I'm trying to learn.

[domenic]

document.implementation.createDocument()

[OnkarRuikar]

To save googling time, here is the completed example https://stackoverflow.com/a/69602658/15273968