Rather than start off explaining the constitution of a proper post, I'd rather explain what an improper post is. An improper post is one that tries to sell a product. It is one that says "test." Such posts should be tested in the newsgroup alt.test. The group is not a political playground. Posts talking about "the racist white police arresting blah blah blah" are unacceptable. Also, do not reply to such posts.
The set of appropriate posts includes questions dealing with web development (especially HTML and CSS), and of course answers (or locations of answers) to those questions. Many of the group's members are quite the corny joke artists, and some jokes might be well received, if it has something to do with its thread. (Do not create a new thread just to tell some stupid joke -- most are stupid.)
Keep your sigs short, and separate them from the rest of your post with a sig delimiter (an line containing only two hyphens and a space: "-- "). Proper posts have the quoted text above the reply. Also, use your real name. Real name.