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percentage heights in tables (section 17.5.3 of the CSS2.1 spec on "table height algorithms")

From: Peter Moulder <Peter.Moulder@infotech.monash.edu.au>
Date: Wed, 17 Mar 2010 12:28:04 +1100
To: sam <samuelp@iinet.net.au>, www-style@w3.org
Message-id: <20100317012804.GA15191@bowman.infotech.monash.edu.au>
On Tue, Mar 16, 2010 at 07:25:17PM -0500, Tab Atkins Jr. wrote:
> On Tue, Mar 16, 2010 at 4:37 AM, sam <samuelp@iinet.net.au> wrote:
> > section 17.5.3 of the CSS2.1 spec on "table height algorithms"
> > (http://www.w3.org/TR/CSS2/tables.html#height-layout) states:
> >
> > "CSS 2.1 does not define how the height of table cells and table rows is
> > calculated when their height is specified using percentage values."
> >
> > Why? Would it not be reasonable for the height of a row to be relative to
> > the height of the table, height of a cell relative to height of its row or
> > sum of the heights of the rows it spans?  [...]
> 
> Often, "does not define" in CSS 2.1 means "browsers did all sorts of
> crazy things, and we decided not to try and stabilize the behavior at
> this time".
> 
> Your suggestions do seem reasonable, I think.

I will just note that it does add a little to the implementation cost, in that
an implementation would prefer to know how high a cell/row is before they know
how high the row/table is.

The fact that it's only a minimum height actually makes things harder, because
it makes it harder to work out which rows heights are a percentage of the table
height, which we need because we want to say "these rows sum to 400px, while
the others sum to 60%, so 400px must be 40% of the table height, so we can
interpret a height of '10%' as '100px'".  Because it's just a minimum, the row
will be determined either by its minimum percentage or minimum fixed height
depending on how tall the table is (i.e. by whether the percentage or the fixed
height is greater), so you pretty much have to sort by the ratio of the two
(i.e. sort by table height cutoff point), and then table layout is no longer
linear time.  However, I believe that's mostly an academic point: I expect
you'd need a really huge number of rows for the sort time to dominate.

The main point is that it does add more implementation effort than you'd
expect, and at the moment I believe there are still more significant issues
with tables both in the spec and in the extent to which the table spec is
implemented in common CSS user agents, so I would expect percentage row heights
to be widely implemented soon.

pjrm.
Received on Wednesday, 17 March 2010 01:28:36 GMT

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