CustomElementRegistry: initialize() with DocumentFragment and Document

[annevk]

No description provided.

[tbroyer]

Is it expected that one could use window.customElements/document.customElementRegistry to initialize() another document?

let doc = new Document();
customElements.initialize(doc);

Should this be tested? (whichever the expected outcome)

[annevk]

Yeah that should work. Feel free to push another commit to this branch.