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

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.

This is important because physical screen dimensions are going to become
very crucial in the near-term for content and layout delivery, especially in
regards to Media Queries.

The problem which was years away is now an impending reality: the day
when 720p+
mobile devices<http://www.engadget.com/2011/09/26/samsung-announces-galaxy-s-ii-lte-and-galaxy-s-ii-hd-lte-handset/>become
the norm.  Unless these devices are all pixel-doubling and
-quadrupling without exception, using Media Queries for the most common
responsive design practice — horizontal resolution detection in pixels —
won't work anymore.  They will simply cease to be a solution, leaving no
replacement.

We need a new way to do generic device detection, or we need absolute length
units to work.

Anyone from representative groups (Mozilla, Chrome Team, Apple) want to
comment?  Does the spec need to be changed?

Best,
-Brian