Currently, UA Widgets reuses the DTDs that gets included when we parse XBL. https://searchfox.org/mozilla-central/rev/50ba1dd30cf013bddce1ae756f1b3c95b26f0628/toolkit/content/widgets/videocontrols.js#2195-2199 It achieves that by having its markup ran through DOMParser (and also leverage the fact that we all any web content to access resource DTDs). UA Widgets ( https://firefox-source-docs.mozilla.org/toolkit/content/toolkit_widgets/ua_widget.html ) are special JS + Shadow DOM dropped to the web content document. The JS holds expanded principal so it's possible to give them access to APIs unavailable to web content. We'll need some setup to enable UA Widgets to get the strings from the browser, without disrupting the content document (e.g. put the strings to a place accessible to the web content or deny its ability to use Fluent independently) Perhaps Fluent can localize the Shadow DOM without walking into the document DOM? Perhaps through some gatekeeping via WebIDL UA Widgets can access Fluent and string resources without letting web content to do the same? This is not a hard blocker to UA Widgets or de-XBL effort. I am filing this bug so I can include this in the datetimebox.js patch.
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