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Re: Another attempt at solving the @page problem (issue 71)

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Wed, 21 Apr 2010 13:39:25 -0700
Message-ID: <m2tdd0fbad1004211339o8770456fqa5c49317478b41ec@mail.gmail.com>
To: Bert Bos <bert@w3.org>
Cc: W3C style mailing list <www-style@w3.org>
On Wed, Apr 21, 2010 at 2:35 AM, Bert Bos <bert@w3.org> wrote:
>  1) Change Paged Media to conform to CSS 2.1. That requires that margin
>    boxes (@top, @top-left, etc.) are be declared outside @page, e.g.:
>
>        @page { margin: 1in }
>        @top { content: "Foo" }
>
>    ADVANTAGE: This is arguably what Paged Media should have been like,
>    because it matches the style of CSS (which avoids nesting as much as
>    possible) and it fits the generic grammar of CSS (which is designed
>    to mix rulesets and at-rules, not declarations and at-rules).
>
>    DISADVANTAGE: There are known to be implementations already of the
>    Paged Media module, since it was a CR once. Those implementations
>    appear difficult to change. I know of Prince, PDFReactor, and the HP
>    Deskjet 990 (although I haven't verified that last one myself).

A further problem with this is that you still need some way to
associate margin rules with pages.  Nesting happens to be a natural
way to do so.


>  2) Change the error recovery rules for declaration blocks in
>    section 4.2 such that an invalid declaration, if it starts with an
>    at-keyword, is considered to end at a right curly brace as well as
>    at a semicolon. A ruleset such as
>
>        p {color: green; @x @y {foo} color: red}
>
>    will then be parsed as
>
>        p {color: green; color: red}
>
>    instead of the currently required:
>
>        p {color: green}
>
>    DISADVANTAGE: It's rather ugly that different invalid tokens require
>    different recovery strategies. But, more importantly, all known
>    implementations will have to change. Even the UAs that implement the
>    Paged Media module ignore the 'color: red' in the above example. An
>    unknown number of (invalid) style sheets on the Web will change
>    meaning. (Which may affect proprietary extensions as well, e.g., if
>    some software defines a meaning for '@if {cond} color: red'. We
>    don't like proprietary extensions, but we *did* promise this
>    parsing rule as far back as the CR of 2004.)
>
>  3) Change CSS 2.1 to conform to the Paged Media module: where it says
>    that @page is followed by a declaration block, say instead that
>    @page is followed by a block with both declarations and at-rules.
>
>    ADVANTAGE: The existing implementations of Paged Media do not need
>    to change.
>
>    DISADVANTAGE: UAs that currently follow CSS 2.1 will have to change,
>    viz., by applying a different grammar to @page.
>
>    However, this only affects style sheets that were already
>    unreliable, because they were parsed differently in different UAs.
>
> In June 2009 we decided[1] for option 3, which changed, it seems, a
> decision of September 2008[2] favoring option 2. Today, option 3 still
> appears to be the only practical solution, because the cost of option 1
> is high and the cost of option 2 even higher.
>
> [1] http://www.w3.org/blog/CSS/2009/06/23/resolutions_69
> [2] http://www.w3.org/blog/CSS/2008/09/10/resolutions_36
>
>
> Proposed new text
> =================
>
> I propose to implement the solution by the following concrete changes to
> the current CR text of CSS 2.1:
>
>  a) In section 13.2[3] change
>
>    >   An @page rule consists of the keyword "@page", followed by an
>    >   optional page selector, followed by a block of declarations.
>
>    to
>
>    |   An @page rule consists of the keyword "@page", followed by an
>    |   optional page selector, followed by a block containing
>    |   declarations and at-rules.
>
>  b) Optionally, we can add a note:
>
>    |   Note: CSS level 2 has no at-rules that may appear inside
>    |   @page, but such at-rules are expected to be defined in level 3.
>
>    [3] http://www.w3.org/TR/2009/CR-CSS2-20090908/page.html#page-box
>
> Furthermore, I suggest the following two enhancements to the text of
> CSS 2.1:
>
>  c) In section 4.2[4] in the fifth bullet, change "Invalid
>    at-keywords" to "At-rules with unknown at-keywords".
>
>    Strictly speaking, this is redundant, but it helps readers.
>
>    In fact, it is not stated with so many words, but the three bullets
>    for malformed declarations, malformed statements and invalid
>    at-keywords are ordered: an invalid declaration is handled by
>    ignoring just the declaration; any remaining unexpected
>    token in the statement is then handled by ignoring the statement;
>    and finally a statement that is well-formed, but starts with an
>    unknown at-keyword, is also ignored.
>
>    Applying the bullets in a different order leads to conflicts or to
>    rules that are never applied. My proposed change avoids that
>    readers have to consider those different orders.
>
> [4]
> http://www.w3.org/TR/2009/CR-CSS2-20090908/syndata.html#parsing-errors
>
>  d) Add explicit error-recovery rules in section 13.2, just above
>    13.2.1:
>
>    |   The rules for handling malformed declarations, malformed
>    |   statements, and invalid at-rules inside @page are as defined in
>    |   section 4.2[link], with the following addition:
>    |   when the UA expects the start of a declaration or at-rule
>    |   (i.e., an IDENT token or an ATKEYWORD token) but finds an
>    |   unexpected token instead, that token is considered to be
>    |   the first token of a malformed declaration. I.e., the rule for
>    |   malformed declarations, rather than malformed statements is used
>    |   to determine which tokens to ignore in that case.
>
>    I'm not sure if this is redundant or not. My guess is that it is
>    not.

I believe that, with these changes, it will still be possible to
backwards-compatibly add @keywords inside of declaration blocks, as
long as they're followed by a ;, correct?

I expect that the grammar I will be shooting for with mixins is
something like "foo { @mixin bar(5px); color: blue; }".  I believe
this will work compatibly with the error-handling rules as currently
defined, and as you are revising them.

~TJ
Received on Wednesday, 21 April 2010 20:40:18 GMT

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