Killing off some especially uptight rules

After some polite debate on IRC I've removed a few of the more persnickety rules that don't really matter much and a few others that just don't need to be spelled out. Unclenching.
Why remove this one? Is that not a thing? The basic idea is, don't ``display:none`` unless that's what you want.

Or is this out of place in this part of the docs?