Intent to prototype HTML:inert

[Alexander Surkov]

/Summary/: HTML:inert is attribute that reflects inert subtree concept
(https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees).
It helps to solve a bunch problem on the web including accessibility
ones. For example, it allows to create hidden but accessible content on
a web page. See explainer with more examples
(https://github.com/WICG/inert/blob/master/explainer.md).

/Bug/: https://bugzilla.mozilla.org/show_bug.cgi?id=921504

/Standard/: https://github.com/whatwg/html/pull/4288 (not yet merged,
awaits for the second implementation)

/Platform coverage/: web

/Preference/: html5.inert.enabled (please let me know if there's a
better match)

/DevTools bug/: no extra effort should be required

/Other browsers/:

* Chrome shipped behind a flag (May 2017,
https://bugs.chromium.org/p/chromium/issues/detail?id=692360)

/web-platform-tests/:
https://searchfox.org/mozilla-central/source/testing/web-platform/meta/inert

/Secure contexts/: no concerns to the best of my knowledge

/Is this feature enabled by default in sandboxed iframes?/ yes

[pepe...@gmail.com]

Great news! Once you have it in Nightly, please don’t forget to update the MDN data. I’m currently trying to fix it for existing behind-the-flag implementations https://github.com/mdn/browser-compat-data/pull/6323