Function depth_rules 

pub fn depth_rules(opts: &Emit) -> String

One rule per depth: its fill and its edge, together.

Depth::Flat emits nothing. A class that sets no properties is a class that means “I thought about this”, which is what comments are for.

A pressed rule rides along with the raised one, because the cascade can carry a state that an immediate-mode renderer has to resolve per call site. That is the one thing this renderer gets for free and the others do not.