Specify focus/tabbing order while in fullscreen

[mherkender]

Currently there doesn't appear to be any guidance on focus/tabbing order while an element is fullscreen. In practice this means that the fullscreen order is identical to the non-fullscreen order, and the user can tab to elements not contained by the fullscreen element. I don't think that this behavior makes a lot of sense from a usability standpoint, since these elements are not visible and cannot be interacted with using other input methods.

Quoting the Web Content Accessibility Guidelines: "People with visual impairments can become disoriented when tabbing takes focus someplace unexpected or when they cannot easily find the content surrounding an interactive element." (https://www.w3.org/TR/UNDERSTANDING-WCAG20/navigation-mechanisms-focus-order.html)

I think the order should be the same as if the elements not contained by the fullscreen element had tabbing disabled by default, as if the fullscreen element was a modal dialog.

[foolip]

We just fixed this in Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=787867. Try if you like the behavior in Chrome Canary.

I think the way to fully define this spec-side is to wire up https://html.spec.whatwg.org/multipage/interaction.html#inert for top layer generally, so that the same logic applies to dialog and fullscreen.