[css-grid] grid-line custom identifier auto?

[ewilligers]

Blink and WebKit reject grid-row-end 1 auto.
Edge and Firefox accept grid-row-end 1 auto and serialize it as 1 auto.

Blink and WebKit reject grid-row-end values span 1 auto, span auto 1 and 1 auto span.
Edge and Firefox accept these and serialize all of them as span 1 auto.

Testing with uppercase makes it clear that span is being accepted as a keyword and auto is being accepted as a custom identifier:
http://jsfiddle.net/ericwilligers/meya2q7d/

All browsers reject the following:
auto 1
span span 1, span 1 span and 1 span span
1 span auto, auto 1 span and auto span 1

The relevant grammar for <grid-line> is

<grid-line> =
  auto |
  <custom-ident> |
  [ <integer> && <custom-ident>? ] |
  [ span && [ <integer> || <custom-ident> ] ]

Are auto or span permitted as a <custom-ident>?

The custom-ident spec may be relevant:

Note: When designing grammars with <custom-ident>, the <custom-ident> should always be "positionally unambiguous", so that it’s impossible to conflict with any keyword values in the property.

I don't see a reading of the spec where span 1 auto would be permissible but not span 1 span.

[mrego]

Some excerpts from Grid Layout spec:

• https://drafts.csswg.org/css-grid/#named-lines

  A line name cannot be span, i.e. the in the production excludes the keyword span.

• https://drafts.csswg.org/css-grid/#line-placement

  In all the above productions, the additionally excludes the keyword span.

Also from CSS Values and Units spec:

• https://drafts.csswg.org/css-values/#custom-idents

  This generic data type is denoted by , and represents any valid CSS identifier that would not be misinterpreted as a pre-defined keyword in that property’s value definition.

There is also a related issue #494 and some old threads in www-style:

• https://lists.w3.org/Archives/Public/www-style/2014Feb/0735.html
• https://lists.w3.org/Archives/Public/www-style/2016Mar/0247.html

[fantasai]

Seems to me the spec is clear that span isn't valid as a <custom-ident>. But it does raise the question of whether auto should be similarly excluded.

[css-meeting-bot]

The Working Group just discussed grid-line custom identifier auto?, and agreed to the following:

• RESOLVED: drop auto from grid line syntax

The full IRC log of that discussion

<dael> Topic: grid-line custom identifier auto?
<dael> github: https://github.com//issues/2856
<dael> Rossen_: eric brought this and fantasai you put this on the agenda
<dael> fantasai: There was an issue against grid asking if the ID auto should be excluded from the places where we use custom ident to rep. named lined. Auto has a special meaning it doesn't resolve to a named line. We exclude span keyword, but not auto.
<dael> fantasai: Question if we should exclude auto in places we exclude span
<dael> Rossen_: I wouldn't put span and auto in same cat. for this behavior. span is forward looking directive and auto is a applied to a single item
<dael> fantasai: You cannot have...if you named a line auto you're just going to have trouble, esp. in level 2 where you can combine auto with a custom ident. You can see the grammar in the issue. If a custom ident can include auto you can write auto1 and have that resolve. I'm not sure allowing that is helpful
<dael> fantasai: Where we have span custom ident we're planning to add auto as a similar pattern. That's a grid 2 issue
<fantasai> https://github.com//issues/796#issuecomment-385547068
<dael> Rossen_: I see your point
<dael> Rossen_: I can live with it
<dael> Rossen_: Anyone from mozilla that's close to grid impl. dbaron do you have an opinion?
<dael> dbaron: No opinion. Have to ask Mats
<dael> Rossen_: It's only our impl that allow auto
<dael> fantasai: I think chances of people naming a line auto is pretty much 0 unless you're a QA person
<dael> Rossen_: How did we end up removing span but not auto?
<dael> fantasai: Don't know. I think there's some syntactic construct where one was ambig. It said span and a custom ident you could do and you want to know that you are consuming a custom ident or the span up front. If you write span span is it a keyword or an ident. So we prob did for parsing
<dael> Rossen_: We allow span int and span custom ident
<dael> fantasai: Yes and auto custom ident will be in L2
<dael> Rossen_: Okay, I'm okay with proposed change
<dael> Rossen_: Any other opinions? Or do we try to resolve?
<fantasai> s/auto custom ident/auto <custom-ident>/
<dael> Rossen_: Objections to dropping auto from grid line syntax?
<dael> RESOLVED: drop auto from grid line syntax