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Re: Making pt a non-physical unit

From: Robert O'Callahan <robert@ocallahan.org>
Date: Tue, 12 Jan 2010 22:51:42 +1300
Message-ID: <11e306601001120151j21bc937j7682e7929f207124@mail.gmail.com>
To: Mikko Rantalainen <mikko.rantalainen@peda.net>
Cc: www-style W3C Group <www-style@w3.org>
On Tue, Jan 12, 2010 at 10:37 PM, Robert O'Callahan <robert@ocallahan.org>wrote:

> On Tue, Jan 12, 2010 at 10:00 PM, Mikko Rantalainen <
> mikko.rantalainen@peda.net> wrote:
>
>> I think that this could be a nice way to fix the current mess. I really
>> don't like the fact that one can use e.g. "mm" in the style to refer
>> something else but SI defined mm (0.001 m). However, I do understand
>> that real world UA implementations cannot really implement the "mm" unit
>> according to SI definition because the amount of *existing* incorrectly
>> authored content.
>>
>
> That is not the main issue. Even if there were no compatibility concerns,
> it still would be wrong to force CSS 1mm to be 1mm on every kind of screen.
> That is simply not very useful. It's only useful for touch interfaces and
> life-size diagrams.
>

In fact the current spec says
"*Absolute length* units are only useful when the physical properties of the
output medium are known."
On the Web, the physical properties of the output medium are generally not
known. Web authors following the current spec would simply not be able to
use these units. (OK, they could use elaborate media queries to apply
absolute length units only for a specific narrow range of output media, but
that's not what we want.)

We can redefine these units so that an absolute length unit gets that exact
length when printed on "normal paper" --- material intended to be held when
read --- and let browsers choose the best way to display these lengths
on-screen, based the device characteristics and user settings, ensuring the
desired ratios of 1in = 96px, 1pt = 4/3px etc. Then Web authors can safely
use these units.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]
Received on Tuesday, 12 January 2010 09:52:14 GMT

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