Actually, I had created this simple Drupal input filter for this site, openlife.cc, that I created when publising the English translation of my book Open Life: The Philosophy of Open Source. I wanted to have a simple way to have footnotes within HTML, without having to number them and format them myself. (Same functionality that is common for any word processor.) Drupal's input filter framework made it very simple to add an fn tag. You can see it at work if you read the online version of my book from the openlife.cc web site.