A Responsive Images approach that you can use today that mimics the proposed picture element using spans, for safety sake. Picturefill works best in browsers that support CSS3 media queries. The demo page references (externally) the matchMedia polyfill which makes matchMedia work in media-query-supporting browsers that don't support matchMedia. matchMedia and the matchMedia polyfill are not required for picturefill to work, but they are required to support the media attributes on picture...