It looks like this on Firefox, Windows: https://imgur.com/zNlEGgK
Same for all of the pointless cookie banners - they could've been UA prompts instead, putting the user in charge of setting a policy ("always trust example.com", "never trust example.net", "accept example.org for this session", etc). But building such prompts into a browser would've been a nuisance in 1997... So we ended up with that nuisance anyway, just enshrined by shortsighted laws, that target each and every website - rather than the three remaining browser engine vendors.
The web "browser" wasn't "intended" for this use case, hence the issue. This could be easily fixed though -- just like cookies.
I explicitly do not want such a thing in many of my HTML-apps, but one could add it with relative ease.