[css-multicol-1][css-flexbox-1][css-grid-1] Clarify that column-* properties only apply to block containers

[SebastianZ]

Currently, the Flexbox and Grid specifications explicitly exclude the column-* properties from applying to flex or grid containers.

As discussed in #1352, those exclusions should be removed from Flexbox and Grid and Multi-column be updated to clarify that these properties only apply to block containers.

Sebastian

[Loirooriol]

It's worth noting that table wrappers are technically block containers. Then, for table elements, should these properties apply to the principal table wrapper box (and thus work) or to the additionally-generated table box (and thus not work)? Analogous for block-level ruby.

[css-meeting-bot]

The CSS Working Group just discussed Clarify that column-* properties only apply to block containers, and agreed to the following resolutions:

• RESOLVED: Columns are properties applied to block containers only

The full IRC log of that discussion

<dael> Topic: Clarify that column-* properties only apply to block containers
<dael> Github: https://github.com//issues/1364
<fantasai> It's possible to workaround the lack of clipping on columns by wrapping the columns entire contents in a block with overflow: hidden. This also addresses the weirdness of the clipping boundary being in the middle of the gap instead of at the near edge.
<dael> Florian: This is not a hard change for the spec. If there's agreement I'll do it.
<dael> Florian: There is behavior agreement, it's a question as to if we should udate multicol to say it or are we happy with where it's defined.
<dael> Rossen_: Opinions?
<dael> Rossen_: Proposed: leave it where it is, don't change.
<dbaron> yeah, seems better for the multicol spec to just say the right thing
<dael> fantasai: It's an error in the spec and needs to be changed. We need an exception for tables where they don't apply to tables or table wrapper box. THat should be called out.
<dael> fantasai: For sure block containers. THe list is really specific and not quite correct
<gregwhitworth> not tables
<dael> Rossen_: You propose to change to it applies to only block containers?
<dael> fantasai: Yeah. Block containers is well defined.
<dael> Rossen_: Reasonable. Other opinions?
<fantasai> fantasai: They weren't when the spec was originally written
<dael> Rossen_: Proposal: COlumns are properties applied to block containers only
<dael> Rossen_: Objections?
<dael> RESOLVED: Columns are properties applied to block containers only

[SebastianZ]

Thank you for fixing this, Florian!

I've created #1557 covering the removal of the now redundant exclusions of the column-* properties from the Flexbox and Grid Layout specifications.

(Please excuse the bunch of merge commits! Something went wrong when committing the changes. 😞 The resulting file changes are ok, though.)

Sebastian