Work around a font size flicker bug Safari

When the font-size is defined on html instead of body, Safari 7 flickers
the font size back and forth when resizing around each font-size change.

Easy workaround is to change the font-size on body instead. I did not look into why this happens because other browsers don't do this, so it just seems like a bug in Safari 7.

Video: http://youtu.be/_36K18JheP4

**Steps to reproduce**

  - Visit http://samhuri.net/f/grid-bug.html in Safari 7 on a Mac
  - Resize your browser window enough to cross a size boundary, 40rem or 64rem

You can check out http://samhuri.net to see the fix applied.