Thanks for this great lesson , It includes a  lot of the geeky stuff that are usually ignored although they are important.

Good link  is  mentioned in this lesson:

http://kyleschaeffer.com/best-practices/css-font-size-em-vs-px-vs-pt-vs/

What is the difference between font-size: smaller and small ? , also between larger and large ?; they look the same in example provided .