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Re: User Agents Do Not Implement Absolute Length Units, Places Responsive Design in Jeopardy

From: L. David Baron <dbaron@dbaron.org>
Date: Wed, 5 Oct 2011 10:55:47 -0700
To: Brian Blakely <anewpage.media@gmail.com>
Cc: www-style@w3.org
Message-ID: <20111005175547.GA3968@pickering.dbaron.org>
On Wednesday 2011-10-05 10:17 -0400, Brian Blakely wrote:
> See spec for a refresh: http://www.w3.org/TR/css3-values/#absolute-lengths
> 
> As far as I know, UAs have never actually implemented this, but always
> pretended to anyway.  If you size something as "1in", you're more than
> likely going to get 90px, regardless of the accuracy of this output.

96px, but yes.

Mozilla browsers used to do things differently:  it tried to get the
pixel density of the device, and used the *larger* of the real one
or 96dpi (since assuming a value smaller than 96dpi frequently led
to illegible fonts due to authors assuming that a 7pt or 8pt font
would be legible, which it isn't due to rasterization at that size).

(The 96dpi size comes from Windows, whose API for determining the
pixel density of the display report results determined (at least in
older versions of Windows) by a preference whose values are "Normal
Fonts" (which means 96dpi), "Large Fonts" (which means 120dpi) and
"Custom".  This meant that the vast majority of Web users had
displays detected as 96dpi, even when they were other sizes.)

But this frequently led to Web-compatibility problems for users on
devices correctly detected as having a pixel density greater than
96dpi, because Web authors tended to mix pixel and physical units
however worked for them, often without understanding what the
difference between a pixel and a point was, and in ways that assumed
that 72pt == 96px.

So Gecko switched to always mapping the existing physical units at
96dpi.  See the posts Boris linked to in
http://www.w3.org/mid/4E8C7F45.3010107@mit.edu .

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂
Received on Wednesday, 5 October 2011 17:56:11 GMT

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