Updating guidelines on unit sizing based on today's discussions

@stuartcharlton, @enginehouse, you missed the latter part about the discussion where I found out relative units are still required for good accessibility practices (around 10% of IE users change their default font size [according to this study](http://archive.oreilly.com/pub/post/more_statistics_on_user_clicks.html)).

I've talked this through with @Koded, @cxmark, and @bastow0, and this is how I suggested updating the standards appropriately. Comments welcome!