Respond.js
A fast & lightweight polyfill for min/max-width CSS3 Media Queries
...However, everything I came across was using pixel values and not percentages meaning I wasn't finding much of anything that could be used on a responsive/flexible layout. This script is the defacto way to enable use of HTML5 sectioning elements in legacy Internet Explorer, as well as default HTML5 styling in Internet Explorer 6 - 9, Safari 4.x (and iPhone 3.x), and Firefox 3.x.