Currently, the only rule is to make at least some gesture towards maintaining thematic coherency when making submissions. And if the story-starter lays down any conventions in the comments, please try to follow them. If you don't like them, start your own story!
Regarding submission mechanics, the submissions are copied essentially verbatim onto the growing story page, so HTML commands such as <i> ... </i> can be embedded in the text with the appropriate effects (don't mess up though). However, paragraph commands are inserted automatically, so no need to put in <p>'s, <br>'s, or what have you, as long as you skip a line between paragraphs so the formatter can recognize them.
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