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RE: [css3-page] LCWD issue 22 -- [22] Section 3.3.2 <length>

From: BIGELOW,JIM (HP-Boise,ex1) <jim.bigelow@hp.com>
Date: Tue, 17 Feb 2004 15:22:47 -0500
Message-ID: <79417AA297C63F4EA33B68AC105464A901BFD3B9@xboi22.boise.itc.hp.com>
To: ernestcline@mindspring.com
Cc: W3C CSS List <www-style@w3.org>

Ernest wrote [1]:
> ... what the size property is describing is not the size 
> of a page sheet which is physical, but the size of a page 
> box.  Granted, the usual intent will be to specify a page box 
> that will occupy a page sheet exactly, but that is not the 
> only use for specifying the size of a page box.  Therefore, 
> an author might wish to specify an abstract page box which 
> the UA will then fit if his intent is to insure that a 
> certain number of lines will be printed per page  The 
> mechanism would in essence be no different from having 
> specified "size:A4" and the UA having to decide how to print 
> the A4-sized page box on a letter-sized page sheet. Thus the 
> physicality of the page sheet is not a relevant point. The 
> only relevant point is "Is there a reasonable interpretation 
> for the relative  units "em", and "ex" with respect to the page box?"

After reading the line of reasoning in [1], I disagree.  

The Chicago Manual of Style defines em as, "The unit of linear measure equal
to the point size of the type in question." The CSS 2.0 and 2.1
specifications define em and ex as relative units. Reference to the width
variability fonts is missing from the reasoning in [1]. The units ex and em
are defined in terms of a font, and so can be different for Serif and
Sans-Serif.

Use of em and ex makes sense in the context of text where the font-family,
font-size, font-weight, font-style, font-stretch, font-variant, line-height
and letter-spacing properties apply to an abstract character sequence [2].
However, both the document (content of body element) and page box can
contain many abstract character sequences with different values of em and
ex.  Requiring the UA to determine the values of em and ex before
determining the size of the page could require multiple passes through the
document.

On another note, I don't see that the document author gains much in clarity
of style sheets or usability of CSS properties by allowing the use of em and
ex. 

Points and picas are units of absolute length rather than relative units.
The other relative unit, px (pixel) is device dependent.  Images printed in
a  300 dot per inch device may not be the same size as one rendered on a 600
dpi device. The same pitfall of device dependence holds for specifying the
page size in pixels.

[1] http://lists.w3.org/Archives/Public/www-style/2004Feb/0296.html
[2] http://www.unicode.org/versions/Unicode4.0.0/ch03.pdf, D4, Page 64.
Received on Tuesday, 17 February 2004 15:22:58 GMT

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