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Re: [CSS21] Proposal for a replacement for section 17.2.1 (table anonymous objects)

From: Arron Eicholz <Arron.Eicholz@microsoft.com>
Date: Wed, 22 Jul 2009 17:45:13 +0000
To: "www-style@w3.org" <www-style@w3.org>
Message-ID: <73677D764AE6DE4D8362C8EC1A677ECC18AF776F@tk5ex14mbxc106.redmond.corp.microsoft.com>
We looked over this proposal and here are some of our questions and comments.

On May 30, 2009, Boris Zbarsky wrote:
> ...
> ===================================================================
> Definitions:
>
> 1) A non-replaced box is called "table-related-inside" if its value
>     of the "display" property is one of 'table', 'inline-table',
>     'table-row-group', 'table-header-group', 'table-footer-group',
>     or 'table-row'.
> 2) A box is called "table-related-outside" if its value of the "display"
>     property is one of 'table-row-group', 'table-header-group',
>     'table-footer-group', 'table-row', 'table-column-group',
>     'table-column, 'table-cell', or 'table-caption'.
> 3) Two boxes which have the same parent box and are both in-flow
>     are "adjacent" if there are no boxes between them in the parent's
>     in-flow list.  This is a little fuzzy, but the entire box tree
>     definition is pretty fuzzy.  I can pin this down more in terms
>     of Gecko's implementation, but that seems ... imperfect.  I
>     welcome ideas on how to make this concept (which I think there's
>     general agreement on amongst implementors) clearer.
> 4) A nonempty set of in-flow boxes is called "consecutive" if all
>     elements of the set have the same parent, and if no box not in
>     the set is adjacent to two different boxes in the set.
>     The parent of a consecutive set of boxes is well-defined: it is
>     the parent of any box in the set.
> 5) For a consecutive set of boxes S, we say "wrap set S in box B"
>     to mean that box B is placed in the in-flow child list of the
>     parent of S immediately before the first box belonging to S and
>     then all boxes in S are removed from their parent's in-flow child
>     list and inserted into B's in-flow child list.  This operation
>     preserves the order of the boxes in S.
> 6) A box is a "valid table child" if it is a 'table-row-group',
>     'table-header-group', 'table-footer-group', 'table-caption',
>     'table-column-group', 'table-column', or 'table-row' box.
> 7) A box is "discardable" if it is generated by a Text node that
>     contains only whitespace.  The value of the 'white-space'
>     property does not affect whether a box is discardable.

Definition of "discardable": this is a little confusing, because the definition is so narrow (i.e., boxes generated solely by whitespace-only text nodes), yet reading through the rules, a lot of boxes can get discarded (e.g., any child of a table-column), and thus would seem to be 'discardable'.

> 8) To "discard" a box means to remove it from the box tree.  It, and any
>     child boxes it might have, are not rendered.

Are the boxes also removed from the DOM tree if they are discarded? I would assume not, but just want to confirm. The term "discard" may be a bit too harsh here. Maybe just say removed or not rendered. Just for clarification, the term 'box tree', what is this specifically referring to?

>
> Anonymous table objects are generated according to the following rules:
>
> 1) For every out-of-flow box T which would be a child of a
>     table-related-inside box P if T were in-flow, insert an inline box
>     with width and height 0 in the child list of P at the position
>     where T would appear if it were in-flow.  If this inline box is
>     ever discarded, also discard the out-of-flow box T.  The inline box
>     is called the "placeholder" of T.
> 2) Discard all child boxes of 'table-column' boxes.
> 3) If a box T is not a 'table-column' box and has a parent P which is a
>     'table-column-group' box, discard T.

> 4) If a box P that is not table-related-inside has an in-flow child box
>     T that is table-related-outside, generate a box P'.  P' must be an
>     'inline-table' box if P is 'inline' and a 'table' box otherwise.
>     Take the maximal consecutive set of boxes which contains T and
>     consists only of boxes that are table-related-outside and wrap this
>     set in P'.
> 5) If a box P is a 'table' or 'inline-table' box and has a child T
>     which is not a valid table child, generate a 'table-row' box P'.
>     Take the maximal consecutive set of boxes which contains T and
>     consists only of boxes that are not valid table child boxes, and
>     wrap this set in P'.  If P' now has only one child and this child
>     is discardable, discard P'.

Rules 5 and 7: why only discard a generated box if it has one discardable child? What if there are multiple discardable children?

> 6) If a box P is a 'table-row-group', 'table-header-group', or
>     'table-footer-group' box and contains a child T which is not a
>     'table-row' box, generate a 'table-row' box P'.  Take the maximal
>     consecutive set of boxes which contains T and consists only of
>     boxes that are not 'table-row' boxes, and wrap this set in P'.
>     If P' now has only one child and this child is discardable,
>     discard P'.
> 7) If a box P is a 'table-row' box and contains a child T which is
>     not a 'table-cell' box, generate a 'table-cell' box P'.  Take
>     the maximal consecutive set of boxes which contains T and
>     consists only of boxes that are not 'table-cell' boxes, and
>     wrap this set in P'. If P' now has only one child and this
>     child is discardable, discard P'.  Otherwise, if P' now has an
>     in-flow child box which is the placeholder for a box S and S
>     has a computed value of 'float' other than 'none', then P'
>     becomes the containing block of S.

Rules 5 and 7: why only discard a generated box if it has one discardable child? What if there are multiple discardable children?

> 8) Continue applying rules 4-7 until a pass through all four
>     rules yields no changes to the box tree.
>
> ===================================================================
>
> Note that rules 4-7 can actually be applied in any order desired, or
> even concurrently, since each rule can only affect the child lists of a
> certain set of boxes and the four sets are disjoint.
>
> An alternate stopping condition for rule 8 is to keep applying rules 4-7
> until there are no "impedance mismatches" or whatever you want to call
> situations with missing boxes; one would then need to define that term.
>   I think the stopping condition used in the proposal above is simpler
> to state and sufficiently well-defined; in practice it's pretty clear
> when one is done with this algorithm.
>
> I have posted some tests for the above algorithm at
> <http://test.csswg.org/svn/submitted/css2.1/tables/>.  The tests do pass
> in current Gecko development builds; all other browsers I have tried
> (including Gecko before I started working on this, of course) fail at
> least some of these, especially the ones that test dynamic changes or
> whitespace handling.  The same tests are available as HTML files one can
> just load at
> <http://web.mit.edu/bzbarsky/www/testcases/table-anonymous-objects/>.
> Sadly, that TLD is on IE8's "pretend to be IE7" list, so those wishing
> to try the tests in IE8 will have to copy them elsewhere.
>
> I welcome any feedback on either the above proposal or the tests; I make
> no guarantees about either of them being bug-free, or about the tests
> correctly testing the rules above, though I did try to make sure of all
> of that.  I'm fairly certain the tests don't test the rules
> exhaustively, of course.

What about orphan captions and table-columns/groups (e.g. with no defined table container)? They are considered table-related-outside, yet there is no explicit rule to fix them up in anonymous tables.

--
Thanks,
Arron Eicholz
Received on Wednesday, 22 July 2009 17:45:56 GMT

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