Add guidance for modal blocking

[SoniEx2]

Modals are the new pop-ups. They trigger the same psychological reactions and cause the same issues.

Actually no, they cause worse issues: they're usually shipped with the browser and trigger privacy weakenings. (e.g. notifications, camera, location, etc.)

So yeah, blocking modals by default would block privacy leaks, which is a good thing.

[annevk]

This is too vague to be actionable. I recommend engaging with https://wicg.io/ to learn how to formulate more actionable proposals.

[SoniEx2]

Just copy the informative notes about opening pop-ups (in window.open we believe) into similar APIs like alert and registerProtocolHandler and whatnot