In addition to the WebKit requirement on iOS, I was curious about guideline 2.5.2, which governs downloading and execution of additional code. (I've worked heavily in this area, so I am really interested in where you're planning on taking this. If you're OK with it we can move this to email so we're not spamming the thread.)
My take on it is that every browsers' behavior by design is to download and execute additional code (JS and HTML). Web extensions are JS and HTML code so there is no change in app behavior. Worst case scenario is we have to turn it off on iOS, but we still get the macOS Webkit browser with webextension support which is still unique. Would love to hear your thoughts, my email is above.
Also unrelated: I know it's a UI demo, but you might want to look into adding some vibrancy in your overlays :)