Intent to ship: CSS column-span property

[Ting-Yu Lin 林庭宇]

As of today, I intend to turn "column-span" on by default for all platforms
in Firefox 71. It has been developed behind
"layout.css.column-span.enabled" preference, and has been enabled on
Nighly-only in bug 1423383.

*Summary: *
The CSS column-span property, when its value is set to "all" makes an
element span across all columns in the multi-column layout.

*Bug to turn on by default: *
https://bugzilla.mozilla.org/show_bug.cgi?id=1426010

*Meta bug where this feature is developed:*
https://bugzilla.mozilla.org/show_bug.cgi?id=616436

*Standard:*
https://www.w3.org/TR/css-multicol-1/#column-span

*Platform coverage:*
Like many other CSS features, column-span has support on all platforms.

*DevTools bug:*
None. Column-span doesn't require special support from the DevTools.

*Other browsers: *
According to caniuse.com, Chrome has shipped unprefixed column-span since
Chrome 50 in 2016, and Safari has shipped this in Safari 9 since 2015.
https://caniuse.com/#search=column-span

*web-platform-tests: *
Those tests with title "multicol-span-all-*" in multi-column test suite.
https://github.com/web-platform-tests/wpt/tree/master/css/css-multicol

*Secure contexts:* N/A

*Note:*
To support the development of column-span, the architecture of the multicol
layout needs to be changed in a nontrivial way. If you see any bug, please
one file under "Layout: Columns" component. Thanks!


Ting-Yu Lin

[Patrick Brosset]

Regarding DevTools support: the new value can be found in the CSS values
auto-complete in the Rules panel, so that's great.
But I just wanted to mention the potential inspector feature that could be
built around multicol layouts. We have leading tools when it comes to grid
support, and I think a similar tool could be done for multicol. Something
that highlights the columns, the gaps between them, and the items
(especially useful when items span multiple columns).
Positioning items in a multicol layout is not nearly as complex as doing
the same in a grid layout, so a tool like this is probably not as
important. But still worth considering at some point.

Kudos for shipping this feature btw! Great work.

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