https://html.spec.whatwg.org/multipage/interaction.html#inert

While a browsing context container is marked as inert, its nested browsing context's active document, and all nodes in that Document, must be marked as inert.

Firefox and WebKit don't do this. Blink does, but I'm removing it since it can be a cross-site leak, and blocks fenced frames since it opens a communication channel.

Note that even if the contents in the nested browsing contexts are not marked as inert, if the browsing context container is inert, they won't respond to mouse interactions, and they won't be reached by sequential navigation.

CC @emilio, @nt1m