Intent to Implement CSS Box Alignment on block containers

[Bradley Werth]

Summary: CSS Box Alignment is broadly available for css-flex containers,
and will soon be available for css-grid containers. Web developers who are
using flex containers to take advantage of "justify-content" or
"align-content" will appreciate being able to apply those styles on block
containers also. Implementing this now will help diminish future web
compatibility problems if site owners are overapplying these styles to
block containers and are expecting continued no-op behavior.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1105571
Link to standard: https://drafts.csswg.org/css-align/#overview
Platform coverage: Desktop and Android
Estimated or target release: TBD
Preference behind which this will be implemented:
layout.css.block-align.enabled
DevTools bug: TBD
Do other browser engines implement this? Chrome positive signals:
https://bugs.chromium.org/p/chromium/issues/detail?id=226252
Tests: TBD tests added to
http://test.csswg.org/suites/css-align-3_dev/nightly-unstable/html/chapter-4.htm

[L. David Baron]

It's good to see this happening.

I think it's important to do because it the CSS Working Group's
current plan to address some very common developer complaints about
CSS, such as "can't do vertical centering". These are already
addressed for flex and grid, because these properties apply to flex
and grid already, and this work will make them apply to blocks as
well.

However, it does have some Web-compatibility risk, because we and
other browsers have already been shipping these properties for a
while for flex and grid, so sites might be using them and starting
to depend on the fact that they *don't* work on blocks.

I'm happy to see this happen sooner rather than later, to reduce
this risk.

-David

--
𝄞 L. David Baron http://dbaron.org/ 𝄂
𝄢 Mozilla https://www.mozilla.org/ 𝄂
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offense.
- Robert Frost, Mending Wall (1914)