Type material into the box, and see the preview below. Use $$ ... $$ or <tex> ... </tex> to delimit mathematics. A blank line means a new paragraph.
Hint: Insert '$$ $$' (or '<tex> </tex>') and type between the delimiters to get instant preview as you type. You can copy and paste the preview into some email clients and other programs.
Firefox only: This version of the page works on Firefox, but probably not on other browsers.