Your tip should include some context to why one might want to use the tip and references to vim help for additional reading. All HTML in descriptions will be escaped for security reasons. If you want to use links, just enter "http://...." and it will be turned into a link. Spaces and returns will be maintained so the general structure of your tip will be as you entered it. If you want to refer to other tips or scripts you should use the syntax "keyword #id" and it will be linked to the correct place. For example: vimtip #6 or vimscript #10.
