A crude polyfill for proposed behavior of the picture element, which does not yet exist, but should. :)
Author: Scott Jehl (c) 2012
Notes: For active discussion of the picture element, see http://www.w3.org/community/respimg/. While this code does work, it is intended to be used only for example purposes until either:
A) A W3C Candidate Recommendation for is released
B) A major browser implements
Demo URL: http://scottjehl.com/picturefill/
Note: The demo only polyfills picture support for browsers that support CSS3 media queries, but it includes the matchMedia polyfill for media-query-supporting browsers that don't have matchMedia.