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[css-style-attr] SVG WG comments on CSS Styling Attributes Level 1

From: Chris Lilley <chris@w3.org>
Date: Wed, 10 Feb 2010 00:26:42 +0100
Message-ID: <1938132429.20100210002642@w3.org>
To: www-style@w3.org
Hello www-style,

These comments relate to CSS Styling Attributes Level 1, W3C Working
Draft 21 January 2010 from the perspective of SVG.

1) Thank you for citing SVG 1.1 as one of the specifications which
allows inline style attributes.

2) In section 3. Syntax and Parsing, the text "Note that following the
CSS2.1 convention, comment tokens are not shown in the rule above." is
given as an informative note, but appears to be normative. It states,
in effect, that the normative grammar given is incomplete. Pleas emake
this statement normative.

3) In section 3. Syntax and Parsing, the actual grammar for a style
attribute, following CSS2.1 chapter 4, appears to be

declaration-list
  : C* S* C* declaration? C* [ ';' C* S* C* declaration? C*]* C*
  ;

where C is the comment production. Is that correct? (Specifically, are
leading and trailing comments allowed, as well as ones between tokens?)

4) However, in In section 3. Syntax and Parsing,

  "The value of the styling attribute must match the syntax of the
  contents of a CSS declaration block"

appears at first sight to contradict CSS 2.1 4.1.7 Rule sets,
declaration blocks, and selectors, which states

  "A declaration block starts with a left curly brace ({) and ends
  with the matching right curly brace (}). In between there must be a
  list of zero or more semicolon-separated (;) declarations. "

The contents could be argued to include the curly braces, therefore.
This might be made clearer either by adding wording to css-style-attr
as follows:

  "The value of the styling attribute must match the syntax of the
  contents of a CSS declaration block, i.e excluding the leading and
  trailing '{' and '}' tokens"

or by adding wording to CSS 2.1 to define the term 'declaration block
contents' as follows


  "A declaration block starts with a left curly brace ({) and ends
  with the matching right curly brace (}). In between are the
  declaration block contents, which must be a list of zero or more
  semicolon-separated (;) declarations. "

5) In section 3. Syntax and Parsing, once the fix in comment 4 is
allpied, its good for clarity that the specification clearly disallows
the '{' and '}' tokens. It does man that scoped style attributes (with
a selector before the '{') are disallowed. We assume that this is
intentional.

6) In section 3. Syntax and Parsing, the text

  "The interpreter must parse the styling attribute's value using the
  same forward-compatible parsing rules that apply to parsing
  declaration block contents in a normal CSS style sheet. See chapter
  4 of the CSS2.1 specification for details. [CSS21]"

defers to CSS 2.1. CSS 2.1 in turn defines, in section 4.1.8 only the
CSS 2.1 properties (naturally, for itself; but an issue, when
referenced by other specifications):

  "A user agent must ignore a declaration with an invalid property
  name or an invalid value. Every CSS 2.1 property has its own
  syntactic and semantic restrictions on the values it accepts. "

As a consequence, this specification appears to exclude CSS3
properties, for example, from appearing in style attributes; if they
occur they must be ignored. Please confirm that this exclusion is
intentional. If it is not intentional, please explain how CSS3
properties and property values may appear in a style attribute and
what governs their syntax.

This problem might be avoided by altering CSS2.1 to say

  "A user agent must ignore a declaration with an invalid property
  name or an invalid value. Every CSS property has its own syntactic
  and semantic restrictions on the values it accepts. "

7) Following the same argument as point 6, this specification appears
to exclude SVG properties from appearing in style attributes. If
intentional, this is unacceptable to the SVG WG and appears to
contradict the mention of SVG 1.1 in the Abstract. If unintentional,
please explain how SVG properties and property values may appear in a
style attribute and what governs their syntax.

8) Assuming that comments 6 and 7 are resolved such that CSS3 and SVG
properties and property values are allowed in style attributes and are
not required to be ignored, then the relevant productions would appear
to be

declaration
  : property ':' S* expr prio?
  ;
property
  : IDENT S*
  ;
prio
  : IMPORTANT_SYM S*
  ;
expr
  : term [ operator? term ]*
  ;
term
  : unary_operator?
    [ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
      TIME S* | FREQ S* ]
  | STRING S* | IDENT S* | URI S* | hexcolor | function
  ;
function
  : FUNCTION S* expr ')' S*
  ;
/*
 * There is a constraint on the color that it must
 * have either 3 or 6 hex-digits (i.e., [0-9a-fA-F])
 * after the "#"; e.g., "#000" is OK, but "#abcd" is not.
 */
hexcolor
  : HASH S*
  ;

This appears to allow unitless values ('number'), for those properties
defined to accept them, and scientific notation, for those properties
defined to accept it. However, that depends on the exact syntax of the
NUMBER token, which is apparently defined in a printed book.

[FLEX]
    "Flex: The Lexical Scanner Generator", Version 2.3.7, ISBN 1882114213.

The flex sourceforge page appears to suggest how NUMBER is interpreted
- C99 decimal constant | C99 hexadecimal constant | C99 octal constant
| C99 floating point constant

http://flex.sourceforge.net/manual/Numbers.html#Numbers

Please confirm that this is the case.

9) In section 4. Cascading and Interpretation
Good clear section. We agree that multiple CSS styling
attributes on a single element would be a poor idea.


-- 
 Chris Lilley                    mailto:chris@w3.org
 Technical Director, Interaction Domain
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Tuesday, 9 February 2010 23:26:50 GMT

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