A few of my favorites:
Rulesets should only ever inherit and add to previous ones, never undo.
Basically, if at any point you’re removing previously-declared styles from an element, you’ve probably applied the styles too early. I’m sure I’ve been guilty of this one.
[IDs] are of no use to anyone and should never be used in CSS.
Only recently have I begun to agree with this vein of thought. There’s no harm in replacing
.page-header, but there are situations where it would be beneficial — especially on larger sites. This certainly doesn’t hurt his argument either.