I like most of these except the one about not using !important.
I use .flat { margin-bottom: 0 !important } to remove the default bottom margin from certain typographical elements (ie. h1,p,ul). In this case where I absolutely know that I mean what I say when I apply that class, it works great.
I'd rather see !important instead of this. This rule would call for quite a lengthy comment explaining just what in the hell the repeated .flat rules are for, and what other rules it's attempting to pave over such that you know when you need to add a 5th .flat as a "fix'.
That seems frighteningly unclear. If you do need the "nuclear" option, why not have that be the case where you get into weird specificity hacks (flat.flat !important /hack/) and leave the common style clear in its intention (.flat !important)?
I'm sure grandparent knows that. The problem with cascadingly using em is that you cannot easily deduce the derived em-value on the nth level (e.g. header > nav > div > a, each with their own em-based font-size; good luck reasoning about a's em-value).
This info should really be on that page... Still if you want to enter this market I hope you will do better because I am afraid that those features and way more are already implemented in sites that do that.
For me shame.css doesn't necessarily contain hacks. In mine you will find less thought out selectors that are added in haste and anything else that doesn't conform to the high standard I set myself.
I use .flat { margin-bottom: 0 !important } to remove the default bottom margin from certain typographical elements (ie. h1,p,ul). In this case where I absolutely know that I mean what I say when I apply that class, it works great.