Re: [WICG/webcomponents] Creating non-ugraded defined custom element : `createElement(tagname, {upgrade:false})` (Issue #1097)

denis-migdal left a comment (WICG/webcomponents#1097)

I just noticed that in the two solutions (template and HTML namespace) :
1. the created `HTMLElement` isn't an `HTMLUnknownElement` (seems also to be the case when built with `.innerHTML`) ;
2. `customElement.upgrade()` has no effect on them if we didn't adopt or insert them in the main document.

I think the (1) behavior is an issue as we might have some code using `instanceof HTMLUnknownElement` to detect unupgraded custom elements. Fortunately, it seems that `:not(:define)` works.


So we need to do something like:
```js
const doc = document.implementation.createDocument(
  "http://www.w3.org/1999/xhtml",
  "html",
  null,
);

function createElement(tagname) {
      const elem = doc.createElement(tagname);
      document.adoptNode(elem);
      return elem;
}
```

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