Rebase typography off of mdn 2

This change results in _way_ too much space within any `<dl>` that contains more than one item:
![image](https://cloud.githubusercontent.com/assets/1522391/4906196/d0d57c5a-6456-11e4-9fe8-03ec962e0784.png)

Did you mean to apply this bottom margin to `<dl>` elements?

(Also, nit: Space after `:`)