[alberth]

> body :not(:is(h1,h2,h3,h4,h5,h6)) {

That seems both clever and hard to maintain over the long run.

[taberiand]

What would make it hard to maintain? (honest question, I'm rather ignorant of CSS)

[alberth]

I like my code to be explicit. Meaning, you are clearly defining explicity what you code should DO.

But using the ":not(:set())" operator, you're making your code implicit. Because you're telling your code what NOT to do (and instead rely on defaults or other side effects to define what will happen to your headings line-height).