While this doesn't need to block shipping in Nightly, I think we should
consider advocating for the focus behaviour to be changed (and changing it
in Firefox if we can get it into the spec) before we ship to release.

The currently specified behaviour (and what both Firefox and Chrome
implement) is to focus the first focusable element in the dialog. However,
there are a few major problems with this, including:
1. This means a tabindex="-1" element could get focus, which is focusable
but not in the tab order. That's particularly strange if this gets focus in
preference to something which *does* participate in the tab order.
2. The first focusable element could be a long way down the page; e.g. a
dialog with a lot of preamble text and a form field or button after that
text. That is particularly problematic for screen reader users because that
will direct their reading cursor to the focused control, so they will
potentially not realise they're missing content above it. It might even
cause the page to scroll visually.

What I (and others in the accessibility community) am proposing is that the
dialog element itself should get focus, unless something within the dialog
has autofocus set, in which case we should focus that. There's a spec issue
for this, but it stalled:

Another concern is that when the dialog is dismissed, focus gets thrown
back to the document. Instead, I think it should be returned to the element
which had focus before the dialog was shown, which is the recommended
pattern for good accessibility of dialogs. I don't think there is a spec
issue for this yet.


On Thu, Jun 25, 2020 at 6:04 AM Sean Feng <sef...@mozilla.com> wrote:

> Intent to Ship (Nightly Only) : Dialog Element
> ​
> Hi All,
> ​
> In bug 1645046 <https://bugzilla.mozilla.org/show_bug.cgi?id=1645046>, I
> intend to turn html5 <dialog> element on by default in Nightly. It has
> been developed behind the dom.dialog_element.enabled preference.
> ​
> Meta Tracking Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=840640
> ​
> This is Nightly only because two things needs adjustment in our
> implementation.
> ​
> 1. The inert element isn't supported (bug 921504 -
> https://bugzilla.mozilla.org/show_bug.cgi?id=921504).
>   - For modal dialog, elements that are not part the dialog should be
> mark as inert, so these elements gain the inert-ness and can't be
> focused. Since we don't have inert supported yet, users could use tab to
> move focus out of the dialog, which is not expected.
>   - Next Step: The implementation of inert element is going to be
> started soon (I think), and we can also discuss to support the
> inert-ness without the supporting of inert element
> ​
> 2. We currently use a temporary solution for the layout of modal dialog.
> (bug 1637310 - https://bugzilla.mozilla.org/show_bug.cgi?id=1637310).
>    - Currently the spec defines modal dialog as an absolute element,
> along with some weird calculation requirement to make the element
> centered. This modal dialog layout felt like a hack to us, so we didn't
> follow it, and instead, we used a temporary solution
> (https://bugzilla.mozilla.org/show_bug.cgi?id=1642364) to make the modal
> dialog as a centered fixed element.
>    - Next Step: The CCSWG agreed to switch modal dialog to be a centered
> fixed element
> (https://github.com/w3c/csswg-drafts/issues/4645#issuecomment-642130060),
> which is the same as the temporary solution we applied in bug 1642364.
> So the temporary solution may become a permanent solution after things
> have been finalized in spec.
> ​
> *Status in other browsers*
> Chrome has it enabled by default since Release 37
> https://www.chromestatus.com/feature/5770237022568448
> *web-platform-tests*:
> https://github.com/web-platform-tests/wpt/tree/master/html/semantics/interactive-elements/the-dialog-element,
> We have them all enabled and passing except for those layout and inert
> related ones.
> *Spec* - https://html.spec.whatwg.org/multipage/#the-dialog-element
> This feature was previously discussed in
> https://groups.google.com/d/msg/mozilla.dev.platform/vTPGW1aJq24/JnEnoH3BEAAJ
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