[tei-council] processing model use of CSS

Decision please. The current description of the processing model tacitly assumes that CSS is used to express output rendition, and it's hard to see how else the job would be done, but the text of the Guidelines is rather vague on the topic. Should we say a) outputRendition *must* be expressed using CSS b) it is *recommended* that CSS be used c) we don't care what you use, but our examples assume you will use CSS If we choose b or c, we will presumably need to add some complexity to the scheme (e.g. to say what has been used instead if it's not CSS)