From the spec:
User agents should ensure, e.g. by means of an overlay, that the end user is
aware something is displayed fullscreen. User agents should provide a means of
exiting fullscreen that always works and advertise this to the user. This is
to prevent a site from spoofing the end user by recreating the user agent or
even operating system environment when fullscreen.
https://fullscreen.spec.whatwg.org/#security-and-privacy-considerations
