I'm working on a script that will help with graceful degradation of html 5 in current browsers. For now it covers correct displaying of sectioning (eg. <article>,<header>,<section>,etc.) and text-level elements (eg. <mark>) in Firefox 2.0- and IE6/7. I also plan on targeting Opera 9+, Firefox 3+, Konqueror 4+, and Safari 3.1+ for fixing other display issues, such as <figure><legend>. I will not support browsers which require a total reparsing and tree rebuilding of the DOM to fix things such as Konqueror 3.5-. The name stands for the first letter of all the major browsers except Opera, btw.
Here's the SF page: https://sourceforge.net/projects/fikshtml/
SVN repo: http://fikshtml.svn.sourceforge.net/viewvc/fikshtml/