W3C home > Mailing lists > Public > www-style@w3.org > December 2009

Re: Numeric precision in CSS numbers

From: Anne van Kesteren <annevk@opera.com>
Date: Wed, 23 Dec 2009 13:12:47 +0100
To: "Simon Fraser" <smfr@me.com>, "www-style list" <www-style@w3.org>
Message-ID: <op.u5d37ll464w2qv@anne-van-kesterens-macbook.local>
On Tue, 22 Dec 2009 20:29:30 +0100, Simon Fraser <smfr@me.com> wrote:
> The section on Numbers in CSS3 Values and Units at  
> <http://www.w3.org/TR/css3-values/#numbers> says nothing about numeric  
> precision, other than some wording about values being restricted to a  
> range for some properties.

Specifications typically do not state such limits as they can grow over  
time and depend on hardware constraints and such.

> For integers, it isn't stated how large values can get before they fail  
> to parse, or fail to round-trip; for floating point values, it isn't  
> specified how many significant digits are retained by parsing, and  
> round-tripping. (By round-tripping, I mean setting the value via the CSS  
> OM, and then reading it back.)

I guess it ought to say something about this, but I'd like it if we can do  
it without mentioning limits.

> For both large integers and floating point values with many decimal  
> places, implementations behave differently. Both WebKit and Gecko fall  
> into scientific notation in some cases (e.g. set 'left' to '12345678px',  
> and reading it back will give you '1.23457e+7px' which fails to parse if  
> you in turn try to use it to set the value); Opera does not. WebKit and  
> Gecko also retain 6 significant digits for floating point values; Opera,  
> on the other hand, rounds to two decimal places.

Using scientific notation seems like a bug to me. (Though dino has  
suggested we adopt it in the past.)

> This is a real-world issue. When we changed our number formatting in  
> WebKit, we broke a site <https://bugs.webkit.org/show_bug.cgi?id=32078>.

Ugh. This no longer really amazes me, but it is still annoying.

> Also, we need to be able to round-trip floating point numbers with high  
> precision for CSS transforms.
> There is a test page here (but note that it doesn't work in Opera  
> because of value limiting):
> <https://bug-30341-attachments.webkit.org/attachment.cgi?id=45276>
> I think there are two things that we need to do:
> 1. Be more explicit about the behavior of round-tripping large integer  
> values, and floating point values with lots of significant digits.

I guess we could impose a minimum. I don't think we want to endorse  
serializing to scientific notation but still require it to fail to parse,  
but I don't think you're asking for that either :-)

> 2. Ensure that the CSSValue replacement in CSS OM allows authors to  
> modify style without going through number/string conversions, and  
> therefore losing precision.

Agreed. That is most certainly the plan.

Anne van Kesteren
Received on Wednesday, 23 December 2009 12:13:28 UTC

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