Spotted a few typos and a mistake in your code. I believe the parent should be positioned relatively since the arrows need to be positioned absolutely.
Also (this is not included in this PR), the code example right under the Content dimensions and margins header doesn't contain enough content to show what you're trying to illustrate. Check out the difference: http://codepen.io/brentguf/pen/yaPvAX.
Thanks for writing the book. :)
awesome, thanks! glad you found it useful and sorry for the delay in getting this in!