"the other thing you can do is type your stuff up in a Mudcat message-posting box, and check the "preview" box. When you hit "submit," you display a copy of your work. You can practice HTML as much as you like this way, and nobody can see your work until you've perfected it."

You will get an error message that some html code is "prohibited" if you try this - this is because Mudcat is set up to deter malicious fiddling to the page - which is itself in html... :-)