@jbalogh has a write up on how to implement GitHub’s Ribbons in pure CSS: Redoing the GitHub Ribbon in CSS
img http://s3.amazonaws.com/github/ribbons/forkme_left_red_aa0000.png http://people.mozilla.com/%7Ejbalogh/ribbon/ribbon.html
Daniel Perez Alvarez has a similar writeup which includes a nice “supported browsers” table. Awesome work!