Blank lines separate paragraphs. Text surrounded by asterisks is italicized. To get a literal asterisk, use \* or **. Text after a blank line that is indented by two or more spaces is formatted as code. Urls become links, except in the text field of a submission. If your url gets linked incorrectly, put it in <angle brackets> and it should work.
This page is a 100% standalone HTML file. Copy the entire source and save as hnpreview.html to use offline.