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RE: [CSS2.1] Grammar for @media versus general block parsing

From: Grant, Melinda <melinda.grant@hp.com>
Date: Thu, 3 Jul 2008 00:41:26 +0000
To: Bert Bos <bert@w3.org>, "www-style@w3.org" <www-style@w3.org>
Message-ID: <763AE400FE923441B74861D534DF254943F45AFFF1@GVW0433EXB.americas.hpqcorp.net>


My thoughts intercalated below.

Best wishes,

Melinda

Bert Bos said:
> On Wednesday 25 June 2008 09:38, Bjoern Hoehrmann wrote:
> > * Justin Rogers wrote:
> > >Supporting such a feature in CSS 3 won't be a problem. The
> > >consideration here is how a CSS 2.1 standards compliant parsing
> > >engine should treat the rule given the current wording of the
> > >specification.
>
> Here is my analysis (with proposals):
>
> First, I checked if
>
>     @media all {
>       @import url(foo.css);
>       DIV { color: red; }
>     }
>
> conformed to the generic, forward-compatible grammar. It
> does. So it is in principle available for a future extension
> and therefore must have a defined fallback. Björn's examples
> with {} instead of ; after the @-keyword are similarly well-formed.
>
> Björn's remark about @page inside @media in level 3 suggests
> that we intended the @-rule to be ignored, but *not* the DIV
> rule after it.

As you may recall, we took the decision here (member-only link): http://lists.w3.org/Archives/Member/w3c-css-wg/2007OctDec/0267.html and announced it here: http://lists.w3.org/Archives/Public/www-style/2007Nov/0119.html.  Per my action item, I modified the css3-page grammar to call out explicitly that @page is allowed in @media.  (I don't remember discussing whether or not to require the semi-colon.)

> That is also consistent with what happens at the top level,
> i.e., outside of @media, in the case of an erroneous @-rule.
>
> I checked if that behavior could be inferred from the CSS 2.1
> spec. It's not entirely clear, but the spec *can* be
> interpreted that way:
>
> Chapter 7, Media Types, says that what is inside @media is a
> style sheet, which implies that anything that can occur at
> the top level can also occur inside @media, unless explicitly
> forbidden by other chapters. In other words: according to
> chapter 7 on its own, the following is fine:
>
>     @media all {
>       @import url(foo);
>       @page {margin: 0}
>       DIV {color: red}
>       @media print {
>         DIV {color: green}
>       }
>     }
>
> Looking at the specific rules for @import and @page, we see
> that there are indeed further restrictions. Section 4.1.5,
> At-rules, says that the @import rule must be ignored. I.e.,
> the above is equivalent to
>
>     @media all {
>       @page {margin: 0}
>       DIV {color: red}
>       @media print {
>         DIV {color: green}
>       }
>     }
>
> I didn't actually find any English text that says that the
> @page and @media must be ignored, but the grammar in appendix
> G claims to be a superset of what CSS 2.1 allows, and as the
> grammar clearly doesn't allow @page and @media inside @media,
> the most reasonable interpretation is that those @-rules are
> to be ignored like the @import rule. That leaves
>
>     @media all {
>       DIV {color: red}
>     }
>
> My conclusion is that CSS 2.1 needs two clarifications:
>
> 1) In chapter 13, Paged media, some text has to say in
> English what is currently only implied by the grammar in
> appendix G, viz., that CSS level 2 ignores @page rules that
> occur inside a block. I propose:
>
>     [Insert in 13.2, just above 13.2.1:] In CSS 2.1, @page rules that
>     occur inside any block are ignored. E.g.,
>
>         @media all {
>           @page {margin: 0}
>           DIV {color: green}
>         }
>
>     is equivalent in CSS 2.1 to
>
>         @media all {
>           DIV {color: green}
>         }
>
> 2) Chapter 7, Media types, similarly needs some text to say
> explicitly that @media in level 2 is only allowed outside of
> any blocks. E.g.:
>
>     [Insert at the end of 7.2.1:] In CSS 2.1, @media rules that occur
>     inside any block are ignored. E.g.,
>
>         @media all {
>           @media print {
>             DIV {color: red}
>           }
>           DIV {color: green}
>         }
>
>     is equivalent in CSS 2.1 to
>
>         @media all {
>           DIV {color: green}
>         }
>
> There is an alternative proposal, which might be better in
> light of the fact that those nested @-rules are going to be
> allowed in level 3. That proposal is to add no
> clarifications, but change the grammar in appendix G, with
> the argument that it is in error, because it forbids
> something that isn't forbidden by any text in the
> specification. The media rule would then become:
>
>     media
>       : MEDIA_SYM S* medium [ COMMA S* medium ]*
>         LBRACE S* [ ruleset | media | page ]* '}' S*
>       ;
>
> A potential practical problem is that it would make current
> implementations that ignore @page inside @media
> non-conformant and would delay CSS 2.1 until we have
> sufficient implementations again.
> (Unless we promise not to test nested @page for now...)

There are at least three shipping implementations that support @page inside @media.  They do not require a separating semi-colon, although every version of the spec since it included a grammar has indicated they should.  So it seems that either the spec or implementations must change...

> > I note that most if not all at-rules have similar problems, e.g.,
> > @page accepts nested at-rules in CSS Level 3, but CSS 2.1
> suggests to
> > parse them like the interior of a ruleset, so you get different
> > behavior for
> >
> >   @page { @other { }  margin: 1em } /* versus */
> >   @page { @other { }; margin: 1em } /* versus */
> >
> > Unless the semi-colon is required (the grammar in css3-page
> suggests
> > it is, but the sub-sequent examples do not specify the
> semi-colon be-
> > tween at-rules nested inside @page rules), since you would
> skip to the
> > following ';' at the same {[(-nesting level when parsing a ruleset,
> > ala
> >
> >   p { color: red; @generic-syntax-forbids-this { }; color: green }
> >
> > (You'd wonder then how to handle @page{@other;;margin:1em) ...).
>
> This seems to be a problem with Paged Media rather than with
> CSS 2.1, but it is certainly one that I hadn't realized before.
>
> CSS 2.1 indeed says that the contents of @page are
> declarations. And thus if CSS level 3 wants to allow margin
> boxes ('@top-left', etc.) inside @page, those will be seen as
> malformed declarations for the purpose of level 2, and will
> be ignored up to the next semicolon (;), not the closing
> curly brace (}).
>
> The grammar in Paged Media confirms CSS 2.1 in that respect,
> because it requires semicolons to separate the margin boxes.
> Adding some semicolons to fix the examples is easy. But those
> examples show how tempting it is to write them without semicolons...
>
> Is it too late for a different syntax for margin boxes? E.g.,
> rewriting example V in 3.5.1:
>
>     @page ::top-left { ... }
>     @page ::bottom-center { ... }
>
>     @page :left ::left-middle { ... }
>     @page :right ::right-middle { ... }
>
>     @page :left ::bottom-left-corner { ... }
>     @page :right ::bottom-right-corner { ... }
>     @page :first ::bottom-left-corner { ... }
>     @page :first ::bottom-right-corner { ... }
>
> The double colons are not nice, but they do remind one of
> pseudo-elements (just like the single colon in ':first'
> reminds one of pseudo-classes). And they actually avoid a
> pair of curly braces, which is worth something, too.

The nested bracketed syntax for margin boxes has been in this spec ever since it was first published nine years ago. It's been through several review cycles, including two last calls and a CR.  Several implementations have been shipping this for years.  Making radical changes now would be very harsh, and I think would send a very harmful message.

In my mind, the viable choice is between keeping the requirement for the separating semi-colon (and thereby risking having the implementations we need) and removing that requirement (which apparently leaves us with forward parsing compatibility issues).

>
> Bert
> --
>   Bert Bos                                ( W 3 C ) http://www.w3.org/
>   http://www.w3.org/people/bos                               W3C/ERCIM
>   bert@w3.org                             2004 Rt des Lucioles / BP 93
>   +33 (0)4 92 38 76 92            06902 Sophia Antipolis Cedex, France
>
>
Received on Thursday, 3 July 2008 00:43:45 GMT

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