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[css3-values] calc() and properties that disallow negative values

From: L. David Baron <dbaron@dbaron.org>
Date: Wed, 16 Dec 2009 19:17:40 -0500
To: www-style@w3.org
Message-ID: <20091217001740.GA6620@pickering.dbaron.org>
For a number of CSS properties, negative values are parse errors.
Some of these, such as 'width', are very important use cases for
'calc()' ( http://dev.w3.org/csswg/css3-values/#the-calc-function ).

(There are also a few properties that disallow negative and zero
values: 'column-count', 'widows', and 'orphans', but they're not as
relevant here since they're all integer-valued properties, to which
calc() doesn't currently apply.)

It's not entirely clear to me how we should deal with negative
values interacting with calc() in such cases.  However, I'd like to
make the following strawman proposal so that we at least have *some*
proposal for handling this:

1. While parsing the contents of the calc(), values are accepted
   even if, were they the lone value of the property, they would be
   rejected due to their sign (or any other range restriction on the
   value of the property).

   For example:
     width: calc(50% + -2px);
   is a valid declaration even though -2px is not a valid value for
   the 'width' property.

2. For properties whose values have restricted ranges, the computed
   value of the calc() expression is clamped to the allowed range.

   For example:
     width: calc(50% - 200px);
   computes to zero when the containing block's width is less than
   or equal to 400px (whereas without the clamping it would become
   negative when the containing block's width is less than 400px).

Note that this proposal applies equally to min() and max(), although
for min() and max() at toplevel (i.e., not inside calc()), there are
other reasonable solutions possible (although they complicate
implementation).

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Thursday, 17 December 2009 01:39:22 GMT

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