[css-grid] Explain why empty `<line-names>` is allowed

[cdoublev]

<line-names> allows an empty set since this commit but there is no meaningfull commit message. I would like to know why [] should be allowed, noting that there is a consensus to remove it when serializing repeat(). Maybe it could be explained in a note.

[Loirooriol]

See https://lists.w3.org/Archives/Public/www-style/2013Nov/0329.html

The current ED defines line names as:
<line-names> = ( <ident>* )
do we really want to allow empty line name lists? I guess we don't, and that should be replaced by "+", shouldn't it?

And https://lists.w3.org/Archives/Public/www-style/2013Nov/0337.html

No, we need empty line lists for subgrids (they can specify their own names in addition to any names they inherit, and need empty line lists to clarify that they're not naming a particular line). It doesn't hurt normal grids to allow empty line lists either; it's just equivalent to omitting the parentheses too.

[cdoublev]

Thanks. I should make a habit of searching in the public archives...

I do not know subgrid well enough to have an opinion on how usefull it is to explicitly define a subgrid with subgrid [] vs. subgrid.

It's now clear to me, but I will let you decide if this issue should be closed or if it should remain open until a related note is added.

[Loirooriol]

The thing with subgrid is that subgrid [] [a] and subgrid [a] are different, you can't just omit empty [], because it will change the position of following line names.

I guess a note might have been useful for level 1 since it has no subgrid, but current work is now level 2 so it's probably not worth it.