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[csswg-drafts] [css-values] Traditional typographic absolute length units

From: Christoph Päper via GitHub <sysbot+gh@w3.org>
Date: Tue, 12 Jul 2016 09:36:35 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-165034761-1468316193-sysbot+gh@w3.org>
Crissov has just created a new issue for 
https://github.com/w3c/csswg-drafts:

== [css-values] Traditional typographic absolute length units ==
[Absolute 
lengths](https://drafts.csswg.org/css-values-3/#absolute-lengths)

I’ve mentioned the [**didot** 
point](https://en.wikipedia.org/wiki/Point_(typography)#French_points)
 (and **cicero**, its pica equivalent) several times on www-style 
during the past few years. Unlike recently added `q`, I’ve never 
really been convinced myself that it would make sense to specify it in
 CSS, although I suggested once in 2011 to [*reserve* the units `dd` 
and 
`cc`](https://lists.w3.org/Archives/Public/www-style/2011Sep/0187.html)
 (among others). 

After I’ve found out that [Antenna House 
Formatter](https://www.antennahouse.com/product/ahf60/docs/ahf-ext.html#ext-unit-conf)
 has added support for `dd` and `cc`, I have been reminded that it 
would have been a good idea to add the unit at least to some optional 
module of CSS, because then we would need to agree on a common 
**standard definition** for it. 

As it turns out, AH claims to have defined it as ` 1dd = 0.01483in`. 
That’s exactly `0.376682mm`. The traditional definition of the didot 
point (based upon pre-metric French inches) indeed converted to 
roughly 0.376 mm, which has been used as an approximation by European 
typesetters, but in the later 20th century it had come to mean exactly
 ⅜ of an millimeter, i.e. `0.375mm` would be an approriate 
contemporary definition.  Just for the record, that’s approximately 
`0.01476in`.

I haven’t found anything about custom units in the documentations of, 
for instance, [Prince](http://www.princexml.com/doc/) and 
[Weasyprint](http://weasyprint.readthedocs.io/en/latest/features.html#css)
 yet, by the way.

These thoughts may apply to some other typographers units as well. 
Most traditional English points, though, including the one used by Tex
 as `pt`, are close enough to the Postscript/CSS definition of 1/72 
inch (`bp` in Tex), so a separate unit seems unnecessary.

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/315 using your GitHub 
account
Received on Tuesday, 12 July 2016 09:36:44 UTC

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