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[CSS21] ambiguity parsing 'list-style'

From: L. David Baron <dbaron@dbaron.org>
Date: Fri, 19 Dec 2008 11:01:36 -0500
To: www-style@w3.org
Message-ID: <20081219160136.GA24044@pickering.dbaron.org>

http://www.w3.org/TR/CSS21/generate.html#propdef-list-style defines
the syntax of the 'list-style' shorthand property to be:
#       [ <'list-style-type'> || <'list-style-position'> ||
#       <'list-style-image'> ] | inherit
Since two of the three properties accept the value 'none', this
definition is ambiguous.  For example, it's not clear whether the
shorthand declaration:
  list-style: outside none;
should be expanded into:
  list-style-position: outside;
  list-style-type: none;
  /* implicit list-style-image: none, the initial value */
  list-style-position: outside;
  list-style-image: none;
  /* implicit list-style-type: disc, the initial value */

The spec tries to clarify this with the sentence:
  # A value of 'none' for the 'list-style' property sets both
  # 'list-style-type' and 'list-style-image' to 'none':
  # ul { list-style: none }
  # The result is that no list-item marker is displayed. 
but that doesn't make it clear whether this behavior of setting both
to 'none' occurs only when there aren't values that are clearly one
or the other, or whether any 'none' always applies to both
properties and thus makes any values for either of the other cause
the declaration to be rejected as an error.

The spec should define exactly which values are accepted for the
'list-style' shorthand.

Some possible ways to resolve this ambiguity:

(1) Say that the value 'none' is not an allowed value of
    'list-style-image' while parsing the 'list-style' shorthand
    property.  This means that any value of 'none' is always a value
    for 'list-style-type', which in turn means that we get the first
    of the two expansions above (as expected), but also that
    declarations like:
      list-style: outside none disc;
    become invalid CSS.

    This is close to what WebKit does, except that WebKit accepts
    two occurrences of 'none' in a value.  (It seems that it never
    accepts 'none' along with another value of 'list-style-type',

(2) Say that when 'none' appears in the value of 'list-style', a
    single 'none' can set the value for both 'list-style-type' and
    'list-style-position', but other values for either property are
    always accepted.  (However, having another value for both would
    have to be rejected, since that would imply a duplicate value
    for one of them.  Probably two nones would be accepted.)

    Since the default value of 'list-style-image' is 'none', this is
    equivalent to saying that when a 'none' could be for either
    'list-style-image' or 'list-style-type' given all the other
    values present, it is preferentially allocated to
    'list-style-type' rather than 'list-style-image'.

    This is what Opera does.

(3) Say that any occurrence of 'none' sets both 'list-style-type'
    and 'list-style-image', and any other occurrence of a value for
    either makes the declaration invalid.

    This might be what the spec currently says, but it doesn't seem
    to match any implementations that I could find.

What Gecko does doesn't make much sense.

(2) seems significantly harder to implement than (1) or (3).

It's not clear to me which values need to be parsed for

See testcase at:
which I haven't yet gotten to work in WinIE7.


L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Friday, 19 December 2008 16:02:15 UTC

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