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Re: [css3-2d-transforms][css3-images] <position> grammar is duplicated or points to the wrong spec

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Tue, 24 Jan 2012 08:04:54 -0800
Message-ID: <CAAWBYDCAk_9pTxzyyuSyQDomnKFh3aKb7C7ekuGs0XcJbRUYTQ@mail.gmail.com>
To: Sylvain Galineau <sylvaing@microsoft.com>
Cc: Brian Manthos <brianman@microsoft.com>, "L. David Baron" <dbaron@dbaron.org>, www-style list <www-style@w3.org>, fantasai <fantasai@inkedblade.net>
On Tue, Jan 24, 2012 at 12:29 AM, Sylvain Galineau
<sylvaing@microsoft.com> wrote:
> [Tab Atkins Jr.:]
>> On Mon, Jan 23, 2012 at 9:34 PM, Sylvain Galineau <sylvaing@microsoft.com>
>> wrote:
>> > I am referring to the pattern in Brian’s email. I think many folks will
>> expect an expression like calc(100% -5px) to resolve at compute time in
>> background-position, just like it would when in width.
>>
>> Did you mean something else?  Percentages do *not* resolve at compute-time
>> in either background-position or width.  They can't - they rely on layout
>> info.
>
> That's simply not always true. If your containing block has a fixed width then
> width:50% will compute to half that width. (So will calc(50%), by your own definition).

Yes, of course.  You can't *always* do that, though, so the value as a
whole is defined to resolve at used-value time.  Look, here's the
Computed Value line from CSS 2.1:

Computed value:  	the percentage or 'auto' as specified or the absolute length

Percentages are "as specified" in the computed value.  They're not
resolved into a length.  So a calc() expression containing a
percentage must *also* remain as specified at computed-value time.


> I also don’t know anyone who has any use for writing calc(50%) instead of 50% so it
> may not be so obvious to people that the two are meant to be equivalent. (Though I agree
> they should be)

It appears you are being inconsistent.  Let me quote the example we're
talking about again, from Brian's earlier message:

Brian said:
> For the example
>       Width: 200px;
>       Height: 400px;
>       Background-position: calc(100% - 5px) calc(100% - 10px);
>       Background-repeat: no-repeat;

For the rest of this email, let us assume that the background-image is
100px by 100px.

Now, first, let's assume that it instead had (1)"background-position:
100% 100%".  This would resolve at used-value time to
"background-position: 100px 300px", placing the image snug in the
lower right.  This is intuitive and expected, as it's been the
behavior since 2.1.

In your quote directly above, you said that you expect "X%" and
"calc(X%)" to be equivalent in behavior.  I assume this means that if
the above example instead had (2)"background-position: calc(100%)
calc(100%);", you would expect it to resolve the same as the last
paragraph, to "background-position: 100px 300px;".  Is this correct?
If not, please clarify your statements.

You also stated that you expect most authors to come to the same
conclusions as Brian in this example.  Brian stated that
(3)"background-position: calc(100% - 5px) calc(100% - 5px);" should
resolve to "background-position: 295px 395px;", 95px away from what
(1) resolves to.

David, on the other hand, asserts that (3) should resolve to
"background-position: 95px 295px;", so that the resolved values are
5px less than what (1) resolves to.

Based on your statements so far, you believe that (2) should resolve
the same as (1), but that adding "- 5px" to (2)'s value should *shift
it by 95 pixels in the opposite direction*.  What part of (3) causes
the sudden jump?

~TJ
Received on Tuesday, 24 January 2012 16:05:47 GMT

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