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[Bug 18500] New: Language about establishing containing blocks is incorrect

From: <bugzilla@jessica.w3.org>
Date: Wed, 08 Aug 2012 10:51:14 +0000
To: public-css-bugzilla@w3.org
Message-ID: <bug-18500-5148@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=18500

           Summary: Language about establishing containing blocks is
                    incorrect
           Product: CSS
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: ASSIGNED
          Severity: normal
          Priority: P2
         Component: Transforms
        AssignedTo: ayg@aryeh.name
        ReportedBy: ayg@aryeh.name
         QAContact: public-css-bugzilla@w3.org
                CC: roc@ocallahan.org, ayg@aryeh.name, cmarrin@apple.com,
                    eoconnor@apple.com, smfr@me.com, dino@apple.com,
                    dschulze@adobe.com


roc reported that our wording about transforms establishing containing blocks
is incorrect:

http://lists.w3.org/Archives/Public/www-style/2012Aug/0248.html

Øyvind agreed, and further pointed out that a "containing block" is a box, not
an element:

http://lists.w3.org/Archives/Public/www-style/2012Aug/0264.html

He suggested that we define precisely how we're modifying the CSS 2.1
requirements, rather than using undefined terms like "establishes a containing
block for positioned descendants".  I suggest wording like the following:

"""
Any value other than ‘none’ for the transform results in the creation of a
stacking context.

The containing block of an element with 'position: fixed' is the padding edge
of its nearest ancestor with a 'transform' other than 'none'.  If there is no
such ancestor, its containing block is as specified in CSS 2.1.

The containing block of an element with 'position: absolute' is determined as
specified in CSS 2.1, except that an ancestor element with 'transform' other
than 'none' is treated the same as one with a 'position' of 'absolute',
'fixed', or 'relative'.
"""

With suitable links to the relevant sections of CSS 2.1.  I think the "is
treated the same as" wording is the best available here, because it's quite
clear but doesn't require duplicating the different handling for inline vs.
block ancestors.


Also, Øyvind points out that we currently have two places where this
requirement is stated, slightly contradictory:

"""
In the HTML namespace, any value other than ‘none’ for the transform results in
the creation of both a stacking context and a containing block. The object acts
as a containing block for fixed positioned descendants.
"""

"""
Any value other than ‘none’ for the transform results in the creation of both a
stacking context and a containing block. The object acts as a containing block
for fixed positioned descendants.
"""

Is is only in the HTML namespace, or not?  I suggest removing the second
entirely, and replacing the first with text like what I give above.


Any objections to my proposed change?

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Received on Wednesday, 8 August 2012 10:51:20 GMT

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