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Re: Correction, or update needed?

From: Chris Lilley <chris@w3.org>
Date: Mon, 25 Apr 2011 19:00:04 +0200
Message-ID: <112678710.20110425190004@w3.org>
To: site-comments@w3.org
CC: Linda A. Walsh <w3@tlinx.org>
Hello Linda,

Its not a question of pt vs. px (and px was never defined as device pixels). I suggest you read the CSS 2.1 specification on length units, which explains that em and ex are relative units and the others are absolute.

"Relative length units specify a length relative to another length property. Style sheets that use relative units can more easily scale from one output environment to another. 

Absolute length units are fixed in relation to each other. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc) and the px unit."

http://www.w3.org/TR/CSS2/syndata.html#length-units

Thus, the advice in the QA guidelines is correct. If a stylesheet uses the em unit, then everything is scaled relative to the default font size that the individual reader has chosen as comfortable for their screen and their eyes.

-- 
 Chris Lilley   Technical Director, Interaction Domain                 
 W3C Graphics Activity Lead, Fonts Activity Lead
 Co-Chair, W3C Hypertext CG
 Member, CSS, WebFonts, SVG Working Groups
Received on Monday, 25 April 2011 17:00:07 GMT

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