Concept: out-of-band styling information encoded into a tag for kind 1.

It seems fairly different from the data other tags carry, but I don't think there's any fundamental reason a multi-character tag couldn't carry that kind of data. (Single character tags shouldn't, because they may be indexed, per NIP-12)

Ack? Nack?

I kind of like out-of-band styling because it can be backwards compatible for plain text systems, and is agnostic about input format (markdown, some kind of WYSIWYG editor, etc).

I know there are several cons, but I find those pros compelling.