MUST NOT use capital letters but for the most important things

[jrudolph]

Seriously, MUST NOT / SHOULD NOT makes the overview hard to read.

[alexandru]

I tried making a distinction between rules that shouldn't be broken and those that can be broken if you know what you're doing. As documented in the Preface, those are keywords.

But heah, you're also right, maybe I'll only make them capitalized.

[jrudolph]

Just saying that writing those keywords all-caps clutters the content for me :)

[q42jaap]

It is fairly common in specifications, I don't mind personally.