I would like to learn the mathematics scripting protocol (Tex, or whatever). In support of that I would like to be able to do some practice with it in the exact environment it will be used, but I don’t want to do it by the sink-or-swim method of inflicting on the community a posting of mine written in my learning-curve version of the protocol. Is there a venue or functionality for “live, but afterwards discarded” practice?
Combining the references in the comments:
Also, you can go to the "ask new question" page and compose a question, which offers a live as-you-type preview -- just don't ever click "submit question" and you'll be fine.