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Re: Official lines Re: Using em or percent for properties that need to change

From: Charles McCathieNevile <charles@sidar.org>
Date: Thu, 19 Aug 2004 22:27:22 +0300
To: "Andy Budd" <andy@message.uk.com>, W <w3c-wai-ig@w3.org>
Message-ID: <opsczxrwm5w5l938@widsith.local>

On Wed, 18 Aug 2004 11:27:03 +0100, Andy Budd <andy@message.uk.com> wrote:

> Sorry, I'm probably not making myself clear.
> I feel that the old WCAG recommendation was very unclear. In some  
> respects then new technique is clearer (at least it talks about em's and  
> % rather than just relative units), but I feel it still need  
> clarification as it's open to a good deal on interpretation.
> When I say that I'm interested in hearing the official WAI line, what I  
> mean is that I'm interested in understanding the reasoning behind the  
> checkpoint fully in order to make the technique less subjective.

OK. My reasoning was that it is easier for someone to change the  
presentation (in the simple and common aspect of font-size and the  
relatively simple and probably more common aspect of window size) if they  
are fluid. As Joe has pointed out many times over the last couple of  
years, I made a mistake in not remembering that px was defined by CSS as a  
relative unit, and therefore forgot to state REALLY explicitly that I  
meant "units relative to window settings", or some such, rather than  
things that browsers were not generally expected to change (like px). I  
did write an erratum request to the working group some time ago, but I  
don't know the status of it. I suspect it is in the giant someday pile.

There are some browsers that have decided that it is a reasonable thing to  
change pixel sizes. My reading of the spec (recall the detail I missed in  
the CSS 2 drafts 5 years ago because I was concentrating on another aspect  
of the spec at the time [1] - if you've read CSS 2) doesn't suggest that  
this is an obvious thing to do, nor does common implementation practice  
(handy though it is as a user). I certainly think you are right that it is  
a good idea to clarify the recommendation.

As you say, it is perfectly possible to have multiple stylesheets. I would  
suggest that one, and to provide what I consider best practice the default  
one, should allow for the easiest possible changes in font size for actual  
users (since I believe that WCAG 1 is actually quite applicable to the  
real world and to needs of actual people, if applied with some thought) -  
by using em or percent as the units. Note that there are some other wierd  
bugs people have written about which mean that browsers like IE (it might  
be badly done, but lots of people use it, and IE 6 when released was  
hailed as miles ahead of anything available when we were writing WCAG 1)  
don't always do exactly what you expect. Such is life in the real world.

I hope that clarifies at least what the thinking was all those years ago,  
and I also hope it clarifies what I think are good interpretations of the  
admittedly not perfectly clear checkpoint. Even in Italy, where the law is  
allegedly based on code, not common law, there is a standar process of  
getting clarification when things are not clear. It's called TUPS for the  
Penal code, and I am glad it exists. This seems a sensible process to go  
through in working out how to apply the guidelines.

As I said earlier, this is all opinion on my part. If you want an  
"official" WAI line (and if you are in Italy you may very well want that,  
since it would be easier than explaining this stuff to the relevant  
officials) you have to ask the WCAG group. I suggest continuing in the  
positive and constructive vein is the best way to get useful answers :-)



It was in reply to Chaals:
>> I interpret it as Tina, John and Patrick have suggested. The argument  
>> that convinces me of the need for fluid layouts is the wide range of   
>> actual screen sizes, which includes window sizes.
Andy again:
> If the presentation information is held within the document, then I  
> would agree. However CSS allows you to serve different stylesheets to  
> different devices. As such you could quite happily create one fixed  
> width layout that works on "screen" and another that works on "handheld"  
> devices.

Charles McCathieNevile         charles@sidar.org
FundaciĆ³n Sidar             http://www.sidar.org
Received on Thursday, 19 August 2004 20:28:17 UTC

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