In a take-two-paces-back look at issues relating to CSS and Atom, Henri Sivonen points out that the content of an entry will usually be a full XHTML document, i.e. complete with <html>, <body>, <head> and <title>. This follows from the use of
content type="application/xhtml+xml". In a response Ken points out that the working spec on content says to drop down to <p>paragraph</p> or similar levels, content needs a
rel="fragment" or similar to indicate that it isn't a full doc. This would appear to leave producer CSS usage clearly defined in respect of XHTML content (possibly the norm), but undefined (as yet) otherwise. It remains to be seen to what extent this is followed by consumers.
Henri's post contains a great caveat procreator line :
"In order to test the implementability of the Atom 0.2
snapshot, I tried to implement the feed to the spec instead of
imitating by viewing sample source."
(Henri provides a spec-correct example with inline XHTML)