Any of the "interaction", "behavioural adornment" specs I've seen (BECSS,
Action Sheets and *CENSORED* 8-), have them together. Personally, I have
no idea why. Post-processing, when everything's mushed together in one big
compound doc, sure. But as a source document that has to be maintained,
referenced, and xpointer-ed into? No thanks.
Yes, there's a huge legacy of existing HTML docs that mix style, content,
and behaviour together. But no, we shouldn't continue to promote that as
best practice, IMHO. The HTML WG has made progress wrt presentation, but
not to behaviour.