This page is designed to test whether browsers trap keyboards (and screen readers) within the fullscreen content when they're in fullscreen mode. This paragraph includes this link, which should not be accessible when the image is in full screen mode.
Observations
Screen Reader / Browser Combos that Work
The following screen reader/browser combinations lock users into the fullscreen content. External content is not accessible when in fullscreen mode.
JAWS 2018 in Chrome 71.x
NVDA 2018.4 in Chrome 71.x
Screen Reader / Browser Combos that do not work
The following screen reader/browser combinations expose content that is outside of the fullscreen element when in fullscreen mode.
JAWS 2018 in Firefox 64.0
JAWS 2018 in Internet Explorer 11
NVDA 2018.4 in Firefox 64.0
MS Narrator in Edge (Windows 10)
VoiceOver in Safari, Mac OS High Sierra 10.13.6
Keyboard Test Results
Chrome 71.x and Internet Explorer 11 restrict keyboard access to fullscreen content.
In Firefox 64.0, keyboard users can shift+tab to the non-fullscreen link and execute it.
In MS Edge (Windows 10), keyboard users can seemingly shift+tab to the non-fullscreen link, but cannot execute it.