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[css21][css3-box] please define "block container element" (was: `overflow: hidden` on tables for border rounding doesn't seem to be supported by the standard)

From: Kang-Hao (Kenny) Lu <kennyluck@csail.mit.edu>
Date: Fri, 30 Mar 2012 20:01:53 +0800
Message-ID: <4F75A0B1.4060309@csail.mit.edu>
To: WWW Style <www-style@w3.org>
CC: Anton Prowse <prowse@moonhenge.net>, Gérard Talbot <www-style@gtalbot.org>, "L. David Baron" <dbaron@dbaron.org>
Context: a three-month-old thread[1] about whether 'overflow' applies to
'table'/'inline-table' and what "block container" is in general.

tl;dr version:

1. We should define the term "block container element"
2. We might want to revisit the question whether a
'table'/'inline-table' counts as a "block container element"
3. We might want to avoid the term "block container" and use either
"block container element" or "block container box" as possible.

[1] http://lists.w3.org/Archives/Public/www-style/2011Dec/thread#msg14

(11/12/03 19:36), Anton Prowse wrote:
> On 02/12/2011 21:52, www-style@gtalbot.org wrote:
>> Øyvind Stenhaug <oyvinds@opera.com> a écrit :
>>
>>> On Fri, 02 Dec 2011 03:47:47 +0100, Gérard Talbot
>>> <www-style@gtalbot.org> wrote:
>>>
>>>> overflow applies to inline-table elements though according to
>>>> http://wiki.csswg.org/spec/css2.1#issue-217
>>>>
>>>> I can't help think that it's kinda weird ... overflow applies to
>>>> inline-tables but not to fixed-width tables. <shrug>
>>>
>>> The text modified due to that issue was since superseded by
>>> <http://wiki.csswg.org/spec/css2.1#issue-120> and
>>> <http://wiki.csswg.org/spec/css2.1#issue-266>. Per the current spec
>>> text, overflow does not apply to 'inline-table' nor 'table'.
>>
>> I do not read such in the issues you list. I do not see where it says or
>> would suggest that overflow does not apply to inline-table.
> 
> Issue 266, my issue 6 contained therein.
> 
> Shame that the only one that didn't get pushed to errata was the one
> that was wrong.  Sorry guys :-(
> 
> In the Applies To line of the 'overflow' property, "non-replaced
> block-level elements, table cells, and inline-block elements" should
> have been changed to "block containers and table boxes" not to "block
> containers".

I've been thinking about this for a while now and now I am not sure if
the above solution is sufficient to resolve all the confusion around
this. The broader problem, as far as I can tell, is this:

  CSS 2.1 uses the term "block container element" throughout without
  actually defining it. It's not even clear if there's a consistent
  and satisfactory definition.

To explain this, note that:

1. CSS 2.1 9.2.1[2] has the following italic terms (presumably
representing definitions) "block-level element", "block-level block" and
"block container box", but notably *not* "block container element"

2. "block container element" is used many times in CSS 2.1 (to be
precise, 11 times). The type of a "Applies to" line is, I would assume,
element, so this increases this to 14 times. (Can we always say "block
container elements" in the "Applies to" line? Now that in CSS 2.1 both
"block container" and "block container elements" are used in the
"Applies to" lines, which is, I think, quite confusing.)

3. We definitely want to make "block container element" include
'list-item', so a definition like "a block container element is an
element that all the non-anonymous boxes it generates are block
container boxes" doesn't work. But it's not clear if s/all/any of/ to
the previous definition would work because various people seem to
suggest that a 'table'/'inline-table' is not a "block container
element". (Note that while a table box is not a block container box, a
table wrapper box is, at least in my mental model.)


I see two possibilities here:

A. Explicitly define "block container element" as an element with the
computed 'display' of 'block', 'list-item', 'inline-block',
'table-cell', 'table-caption'.

This has minimal impact to the conformance requirement. It's not too
clear to me how you establish the relationship of "block container
element" and "block container box" though.

B. Define "block container element" with the invariant "a block
container element is an element that at least of the non-anonymous boxes
it generates is a block container box".

This will include 'table'/'inline-table' in the "block container
elements" category and we'll need to review various occurrences of this
term in CSS 2.1.

I think the only place where this does make some differences is whether
::first-letter/::first-line applies[3]:

  # The :first-line pseudo-element can only be attached to a block
  # container element.

  # The :first-letter pseudo-element applies to block container
  # elements.

All browsers I tested don't apply '::first-line/::first-letter' to
'table' (with this test case:

data:text/html,<style>table::first-line { color: red;
}</style><table><caption>should the first line be
red?</caption><tr><td></td></tr></table>

), but I don't see a test like this in the test suite either, so I think
this option is my preference. Why do we want to exclude
'table'/'inline-table' from "table container elements" again?

[2] http://www.w3.org/TR/CSS2/visuren.html#block-boxes
[3] http://www.w3.org/TR/CSS2/selector.html#pseudo-element-selectors

(11/12/02 4:39), L. David Baron wrote:
>> Right now, answering the question "What elements does the 'overflow'
>> property apply to?" is rocket science.  It says it applies to "block
>> containers", but the text isn't a link, and actually finding the
>> definition of that term, assuming there is one at all, is
>> near-impossible.
>
> I think the definition it's intending to refer to is the second
> sentence of this:
>
>   # Except for table boxes, which are described in a later chapter,
>   # and replaced elements, a block-level box is also a block
>   # container box. A block container box either contains only
>   # block-level boxes or establishes an inline formatting context
>   # and thus contains only inline-level boxes. Not all block
>   # container boxes are block-level boxes: non-replaced inline
>   # blocks and non-replaced table cells are block containers but not
>   # block-level boxes. Block-level boxes that are also block
>   # containers are called block boxes.
>    --http://www.w3.org/TR/CSS21/visuren.html#block-boxes
>
> which is admittedly rather imprecise (since it defines the
> characteristics of a block container box rather than defining what
> causes a box to be one), but I think it's pretty clear that tables
> are not block container boxes, and therefore, according to CSS 2.1,
> the 'overflow' property does not apply to tables.

If the definition of a "table container box" is based on the
characterisc that it "either contains only block-level boxes or
establishes an inline formatting context and thus contains only
inline-level boxes", we soon run into circular definition because the
following section, 9.2.1.1 Anonymous block boxes[4], has

  # In other words: if a block container box (such as that generated
  # for the DIV above) has a block-level box inside it (such as the P
  # above), then we force it to have only block-level boxes inside it.

. So the definition of a "block container box" must be "what causes a
box to be one" and hence consists of the piecewise
statement-of-fact-like texts that are spread everywhere:

1. Except for table boxes, which are described in a later chapter, and
replaced elements, a block-level box is also a block container box.

2. Not all block container boxes are block-level boxes: non-replaced
inline blocks and non-replaced table cells are block containers but not
block-level boxes.

3. Floats, absolutely positioned elements, block containers (such as
inline-blocks, table-cells, and table-captions) that are not block
boxes... (to make sure 'table-caption' is included, but I think
including 'table-caption' in block-level elements would probably be a
better solution)

(I think it's probably no longer useful to criticize this text, but can
we link "table box" to 17.4 Tables in the visual formatting model in the
errata so that a CSS 2.1 newbie like me won't confuse "table box" with
"the thing generated by a 'table'"?)

[4] http://www.w3.org/TR/CSS2/visuren.html#anonymous-block-level


By the way, css3-box has this text[5]:

  # In the above, a block container box is, informally, a box that can
  # contain block boxes. More precisely: any box generated by a
  # (pseudo-)element with a computed value for ‘display’ of
  # ‘block’, ‘inline-block’, ‘table-caption’, ‘table-cell’,
  # ‘list-item’ or template.

which is not accurate for 'list-item' because it generates inline marker
boxes too.

Also, css3-box doesn't define "block-level element".

[5]
http://dev.w3.org/csswg/css3-box/#block-level-boxes-containing-blocks-flow


Cheers,
Kenny
Received on Friday, 30 March 2012 12:02:26 GMT

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