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EM vs. REM?

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From: deborah.kaplan
Date: May 30, 2013 10:18AM

I'm currently working with our web designers to put together an
easy-to-use set of documentation for our workplace on
accessibility best practices for all of the different departments
that create websites. During the process, one of the web
designers has introduced me to the CSS3 "rem" unit:

http://www.w3.org/TR/css3-values/#rem-unit
http://www.css3files.com/2012/10/11/relative-is-the-new-absolute-the-rem-unit/

My usual advice up until this point has always been use EM
instead of PX for sizing, as a basic accessibility best practice.
I haven't seen anyone on any of the accessibility forms talk
about REMs. As far as I can tell from various websites I see
around, it seems to be getting primarily used by people who
resisted switching from pixel dimensions to relative at all, as a
more comfortable way of switching. Is there accessibility best
practice about when to use REMs?

Thank you so much,

-Deborah Kaplan
--
Accessibility Team Co-Lead
Dreamwidth Studios, LLC