Our implementation of dialog isn't perfect yet, there are 2 pieces that are missing.

1. The inert element isn't supported (bug 921504).
2. We currently use a temporary solution for the layout of modal dialog, we need some feedback from CSSWG to implement a finalized solution (bug 1637310).

Despite these 2 pieces, I think dialog is in a state where we can enable it in Nightly.

Could you explain how our layout differs from other browsers?
And Is there a plan to support inert? (I guess technically bug 921504 isn't needed, but inert-ness itself.)

The main difference is our modal dialog is fixed, so it remains in centred during scrolling, and it's position: absolute in Chrome.

Well, the current plan (or what I am thinking) is to wait for bug 921504, however, I am open to discuss if we want to implement the inert-ness first to avoid waiting for bug 921504.

So there is this comment https://bugzilla.mozilla.org/show_bug.cgi?id=921504#c16
And https://bugzilla.mozilla.org/show_bug.cgi?id=1200896#c5
So it does sounds like we might need inertness as such even before possible inert attribute.

Marco, would you mind confirm that it's okay to enable dialog in Nightly from an a11y perspective?

Double-checking the team is okay with https://bugzilla.mozilla.org/show_bug.cgi?id=1200896#c5.

Thanks!

Yes, it is OK to enable this now, it gets exposed to accessibility APIs and therefore assistive technologies.

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&revision=b33756c2e33413bae06a10525222254415de0fb9&selectedTaskRun=VMAYOXWwSCqZQ_jUhWvhow.0

Failure log: https://treeherder.mozilla.org/logviewer.html#?job_id=307551465&repo=autoland
https://treeherder.mozilla.org/logviewer.html#?job_id=307550857&repo=autoland
https://treeherder.mozilla.org/logviewer.html#?job_id=307549962&repo=autoland
https://treeherder.mozilla.org/logviewer.html#?job_id=307545509&repo=autoland
https://treeherder.mozilla.org/logviewer.html#?job_id=307554847&repo=autoland

Full range on test failures here.

Backout link: https://hg.mozilla.org/integration/autoland/rev/ee812b6359befd98993f39f8cffd6735ebcd5644

Backed out changeset 397ad8a55255 (Bug 1645046) for causing multiple wpt failures CLOSED TREE

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedTaskRun=T-jaLbugQsmnsZUkPR1YZA.0&resultStatus=testfailed%2Cbusted%2Cexception&revision=397ad8a55255d9471f3d39ec42e63b7bdfea80f6

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307922577&repo=autoland&lineNumber=4779
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307922591&repo=autoland&lineNumber=2195
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307926438&repo=autoland&lineNumber=13115
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=307922583&repo=autoland&lineNumber=5851

Backout: https://hg.mozilla.org/integration/autoland/rev/3a269857b1182b8cf53730ac46ff490839825404

push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&resultStatus=testfailed%2Cbusted%2Cexception&searchStr=wpt&revision=dfe65c0d1d920d8b6f608b8dce09ebdd0aaa98bf

failure log: https://treeherder.mozilla.org/logviewer.html#?job_id=308506383&repo=autoland

backout link: https://hg.mozilla.org/integration/autoland/rev/86302753c7774ad2d31925ee37fe798bb72b242b

[task 2020-07-03T15:15:59.253Z] 15:15:59     INFO - mozversion platform_version: 80.0a1
[task 2020-07-03T15:16:09.694Z] 15:16:09     INFO - Closing logging queue
[task 2020-07-03T15:16:09.696Z] 15:16:09     INFO - queue closed
[task 2020-07-03T15:16:09.696Z] 15:16:09 CRITICAL - Test harness output was not a valid structured log message: 
[task 2020-07-03T15:16:09.696Z] 15:16:09 CRITICAL - Traceback (most recent call last):
[task 2020-07-03T15:16:09.697Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/runtests.py", line 16, in 
[task 2020-07-03T15:16:09.697Z] 15:16:09 CRITICAL -     rv = wptrunner.main()
[task 2020-07-03T15:16:09.697Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 412, in main
[task 2020-07-03T15:16:09.697Z] 15:16:09 CRITICAL -     return start(**kwargs)
[task 2020-07-03T15:16:09.697Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 396, in start
[task 2020-07-03T15:16:09.698Z] 15:16:09 CRITICAL -     rv = not run_tests(**kwargs) or logged_critical.has_log
[task 2020-07-03T15:16:09.698Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 187, in run_tests
[task 2020-07-03T15:16:09.698Z] 15:16:09 CRITICAL -     **kwargs)
[task 2020-07-03T15:16:09.699Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wptrunner.py", line 89, in get_loader
[task 2020-07-03T15:16:09.700Z] 15:16:09 CRITICAL -     chunker_kwargs=chunker_kwargs)
[task 2020-07-03T15:16:09.700Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/testloader.py", line 245, in __init__
[task 2020-07-03T15:16:09.700Z] 15:16:09 CRITICAL -     self._load_tests()
[task 2020-07-03T15:16:09.700Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/testloader.py", line 308, in _load_tests
[task 2020-07-03T15:16:09.701Z] 15:16:09 CRITICAL -     for test_path, test_type, test in self.iter_tests():
[task 2020-07-03T15:16:09.701Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/testloader.py", line 301, in iter_tests
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -     yield test_path, test_type, self.get_test(manifest_file, test, inherit_metadata, test_metadata)
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/testloader.py", line 261, in get_test
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -     return wpttest.from_manifest(manifest_file, manifest_test, inherit_metadata, test_metadata)
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py", line 655, in from_manifest
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -     return test_cls.from_manifest(manifest_file, manifest_test, inherit_metadata, test_metadata)
[task 2020-07-03T15:16:09.702Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py", line 425, in from_manifest
[task 2020-07-03T15:16:09.703Z] 15:16:09 CRITICAL -     quic=quic)
[task 2020-07-03T15:16:09.703Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py", line 400, in __init__
[task 2020-07-03T15:16:09.703Z] 15:16:09 CRITICAL -     path, protocol, quic)
[task 2020-07-03T15:16:09.703Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py", line 168, in __init__
[task 2020-07-03T15:16:09.703Z] 15:16:09 CRITICAL -     self.environment = {"protocol": protocol, "prefs": self.prefs, "quic": quic}
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/wpttest.py", line 325, in prefs
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -     meta_prefs = meta.prefs
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/manifestexpected.py", line 444, in prefs
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -     return prefs(self)
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -   File "/builds/worker/workspace/build/tests/web-platform/tests/tools/wptrunner/wptrunner/manifestexpected.py", line 93, in prefs
[task 2020-07-03T15:16:09.704Z] 15:16:09 CRITICAL -     rv = dict(value(item) for item in node_prefs)
[task 2020-07-03T15:16:09.705Z] 15:16:09 CRITICAL - ValueError: dictionary update sequence element #0 has length 1; 2 is required
[task 2020-07-03T15:16:09.883Z] 15:16:09    ERROR - Return code: 1
[task 2020-07-03T15:16:09.885Z] 15:16:09    ERROR - No checks run.
[task 2020-07-03T15:16:09.885Z] 15:16:09    ERROR - No suite end message was emitted by this harness.
[task 2020-07-03T15:16:09.885Z] 15:16:09 CRITICAL - # TBPL FAILURE #
[task 2020-07-03T15:16:09.885Z] 15:16:09  WARNING - setting return code to 2

https://hg.mozilla.org/mozilla-central/rev/665f8f1a79a0

== Change summary for alert #26391 (as of Wed, 01 Jul 2020 03:24:23 GMT) ==

Improvements:

19% espn fcp android-hw-g5-7-0-arm7-api-16 opt cold 7,641.66 -> 6,161.42

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=26391

MDN docs completed for this change; see https://github.com/mdn/sprints/issues/3503#issuecomment-669868519 for full details.