The patch we did in bug 1637310 was hacky and we were not happy with the solution. There will be discussion in CSS workgroup to talk about better solutions.

In the meantime, we can add a temporary solution for it to allow us to enable it in Nightly for dogfooding.

Spec: https://html.spec.whatwg.org/multipage/#centered-alignment

The spec is opaque about the centered alignment mode and we couldn't
come up a satisfied solution by using the current version of the
spec. There will be discussion in CSSWG to talk about better solutions.
In the meantime, we are adding this patch as a temporary solution with
basic centered alignment support to allow us enabling the feature
in Nightly for more testing.

Backed out for wpt failures on abspos-dialog-layout.html

Backout link: https://hg.mozilla.org/integration/autoland/rev/f912d56347dacf3806370a419d8bc554c0849a72
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=304750217&repo=autoland&lineNumber=14243

Backed out changeset 3d06403ffd5c (bug 1642364) for WPT failures in /semantics/interactive-elements/the-dialog-element/abspos-dialog-layout.html. CLOSED TREE

Log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=304939436&repo=autoland&lineNumber=35386

Push with failures:
https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&revision=3d06403ffd5cea9cd14680acb434bfe3b85cbde2

Backout:
https://hg.mozilla.org/integration/autoland/rev/c8bf3e906cc575195b788cb8533941305fe7f137

https://hg.mozilla.org/mozilla-central/rev/979a3003f357