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Re: [CSS3] @font vs @background

From: L. David Baron <dbaron@dbaron.org>
Date: Sat, 6 Feb 2010 23:43:14 -0800
To: www-style <www-style@w3.org>
Message-ID: <20100207074314.GA6086@pickering.dbaron.org>
On Saturday 2010-02-06 23:02 -0800, Andrew Fedoniouk wrote:
> Grammar of all properties in CSS 2.1. obey this simple set of rules:
> 
> <VALUE_OR_PAIR>
> 	: <VALUE>
> 	| <VALUE> "/" <VALUE>
> 
> <VALUE_COMMA_LIST>
> 	: <VALUE_OR_PAIR>
> 	| <VALUE_COMMA_LIST> "," <VALUE_OR_PAIR>
>         ;
> 
> <VALUE_LIST>
> 	: <VALUE_COMMA_LIST>
> 	| <VALUE_LIST> S+ <VALUE_COMMA_LIST>
> 
> <VALUE>
>         : NUMBER S*
>         | PERCENTAGE S*
>         | LENGTH S*
>         | EMS S*
>         | EXS S*
>         | ANGLE S*
>         | TIME S*
>         | STRING S*
>         | IDENT S* | URI S* | hexcolor | function
> 
> <PROPERTY-DECL>
>         : <NAME> ':' <VALUE_LIST> ';'
>         | <NAME> ':' <VALUE_LIST> <IMPORTANT> ';'


Counterexamples from CSS 2.1:

  font-family: Times New Roman, serif;
  ...this is (Times New Roman), (serif), not (Times) (New) (Roman, serif)

  font: bold italic 1em/1.2 Times New Roman, serif;
  ... this is (bold) (italic) (1em)/(1.2) ((Times New Roman), (serif))

I'd note that in CSS 2.1:

  The only properties using commas as separators are 'cursor',
  'font-family', 'voice-family' and the 'font' shorthand (within the
  'font-family' values).

  The only property using '/' to separate values is the 'font'
  shorthand.

  On the other hand, space-separated values are used in all
  shorthands and also in 'azimuth', 'background-position',
  'border-spacing', 'content', 'counter-increment', 'counter-reset',
  'font-family' (within single font names), 'play-during',
  'text-decoration', and 'voice-family' (like 'font-family').

Thus there are three properties from which you can determine
operator precedence:  'font-family', 'font', and 'voice-family'.
'font' is pretty much useless, however, since it uses space both
above and below comma, and it uses '/' and (outer) space at equal
precedence.

So I don't think you can determine a clear operator precedence order
from CSS 2.1, other than perhaps (based on 'font-family' and
'voice-family') that comma-separated things contain space-separated
things (the opposite of what you describe, but I think quite
logical).

I think if anything it's the 'font' shorthand that's got the broken
syntax, but that's been broken (internally contradictory precedence)
since CSS1.  I think it's also the one that's confusing for authors:
it recently took me quite a while to spot the mistake in:
  font: 1em normal normal Arial, serif;
(Note: most people don't have a font called 'normal normal Arial'.)

I think the general rule that:
  a b, c d
represents groups like this:
  (a b) (c d)
rather than like this:
  a (b c) d
makes sense, and we should stick to it, leaving 'font' as an
exception.

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Sunday, 7 February 2010 07:43:43 GMT

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