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Re: [css3-values][css3-transforms] relative length in functional notation

From: Kang-Hao (Kenny) Lu <kennyluck@csail.mit.edu>
Date: Wed, 04 Jul 2012 15:09:42 +0800
Message-ID: <4FF3EC36.1070005@csail.mit.edu>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
CC: WWW Style <www-style@w3.org>
(12/04/24 3:55), Tab Atkins Jr. wrote:
> Issue 2: calc() and attr() need to resolve values that are normally
> changed at computed-value time
> Closed as Accepted - the spec now says "The computed value of a calc()
> expression is the expression with all components computed, with all
> multiplication/division subexpressions resolved, and with all
> addition/subtraction subexpressions resolved where both sides are the
> same type."
> 
> Let us know if this is acceptable!

Sorry but I just realized the current wording is sort of vague when
talking about a corner case: 'height: calc(0*0%)'. It is treated as
'height: auto' in IE9, Firefox13 and a Chrome version with
'-webkit-calc', similar to 'height: 0%', not 'height: 0'.

Therefore, the note

  # Thus, the computed value of a ‘calc()’ expression can be
  # represented as either a number or a tuple of a dimension and a
  # percentage.

is sort of inexact in that it doesn't say '0' and 'undefined' are different.


The spec should provide a hint to css3-flexbox too. calc(0*0%) should
probably be treated as indefinite if resolved against an indefinite length.


Also, I think "same type" in the quoted prose here should be elaborated
a little bit. The rest of the description of 'calc()' talks about
"resolved type", but both <percentage> and <length> have the same
"resolved type" for the cases we care.

Proposed wording (just a try):

  | The computed value of a calc() expression is the expression with
  | all components computed with all multiplication/division
  | subexpressions resolved, and with all addition/subtraction
  | subexpressions resolved where both sides are the the same type,
  | unless one of which is <percentage> (which doesn't compute to an
  | absolute unit) and the other isn't. User agents must track whether
  | a <percentage> is contained a calc() with a flag.
  |
  | If 'height' is specified with calc() containing a <percentage>
  | , the value must be treated as 'auto'. A length specified with
  | calc()  containing a <percentage> when the <percentage> is resolved
  | against an indefinite length is an indefinite length. Otherwise,
  | it's definite.


A somewhat irrelevant nit picking:

  # At ‘,’, ‘+’, or ‘-’, check that both sides have the same
  # type, or that one side is a <number> and the other is an <integer>.

',' is no longer relevant since we dropped max() and min().


Cheers,
Kenny
Received on Wednesday, 4 July 2012 07:10:09 GMT

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