Would it be reasonable to export https://html.spec.whatwg.org/multipage/browsers.html#creating-a-new-top-level-browsing-context for use by W3C specs? This is referenced by a handful of web app related algorithms:
https://www.w3.org/TR/appmanifest/#launching-a-shortcut
https://w3c.github.io/web-share-target/#launching-the-web-share-target
https://wicg.github.io/manifest-incubations/index.html#dfn-launch-a-web-app
https://wicg.github.io/manifest-incubations/index.html#handling-a-protocol-launch
https://wicg.github.io/manifest-incubations/index.html#launching-the-new_note_url
https://wicg.github.io/web-app-launch/#preparing-a-new-web-app-launch-client

(Context: w3c/manifest#1056 (comment))