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Re: [css3-syntax] Separation of concerns: "declaration/rule-filled", "can be made important"

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Sat, 26 Jan 2013 21:01:43 -0800
Message-ID: <CAAWBYDAoGBKGAu_rzsBmZE_V7xRJud+e9SfghgyzS7+p8WvdQw@mail.gmail.com>
To: Simon Sapin <simon.sapin@kozea.fr>
Cc: www-style list <www-style@w3.org>
On Sat, Jan 26, 2013 at 10:03 AM, Simon Sapin <simon.sapin@kozea.fr> wrote:
> Hi,
>
> I think that CSS tokenization and tree construction should not depend on
> complex knowledge of other CSS modules. This is basic separation of of
> concerns, and our current charter also suggests that CSS syntax should be
> independent from the rest of CSS:
>
> http://www.w3.org/2010/09/CSSWG/charter
>
> I find two points in tree construction as it currently stands where this is
> not quite the case:
>
> 1. How to decide whether a given at-rule is "declaration-filled",
> "rule-filled" or neither.
> 2. How to decide whether a given declaration "can be made important".
>
>
> Is it guaranteed that an at-rule’s name (its at-keyword) is and will even be
> enough to decide 1.? In other words, 1. does not depend on the at-rule’s
> context/parent or its prelude.
>
> If this is the case, tree construction could take a list of at-rule names
> that information[1] as part of its "input", in addition to a list of tokens
> and a quirks mode flag. The introduction of §3.5. Tree Construction should
> be updated in that direction.
>
> [1] A mapping of at-rule names to declaration-filled/rule-filled/neither

This is already assumed by the spec - you have to know whether the
rule is rule-filled or declaration-filled by the time you hit the
corresponding { token, so at *most* it could depend on the at-rule's
name and maybe its prelude.  In practice, the name is enough.

I'll try and make this more explicit.


> As to declarations that can be made important, I see two options:
>
> a. Specify that style rules contain property declarations (that can be made
> important). For at-rules, refine the "declaration-filled" concept into
> "property-filled" or "descriptor-filled". This implies that at-rules can not
> contain both property declarations and descriptor declaration, and that this
> can be decided based on the name (at-keyword) only, as above.
>
> b. Get rid entirely of the "can be made important" concept. Leave it to
> higher-level parsers to reject a declaration (just like `width: red` is
> rejected as invalid) if it has the important flag but can not be made
> important.
>
> I’m not aware of a anything in CSS that uses the style rule syntax (not
> at-rule) and contains descriptors (that can not be made important), but note
> that a. prevents using such syntax in the future.

Descriptors only exist in at-rules so far (and likely forever), but
keyframe blocks are style rules whose contents can't be made
important.

However, I don't think I gain anything from going further.  I mean,
the check for important-capableness has to occur in the parser.  The
latest I can push it to is when the rule is popped and syntax-checked.
 That doesn't really change anything.

~TJ
Received on Sunday, 27 January 2013 05:02:30 GMT

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