Matt Marquis, chair of the Responsive Images Community Group, asked on Twitter:
Anyone know whether
picture/Picturefill support is officially in the works for WordPress? Could swear I saw something about that once.
https://twitter.com/wilto/status/468442998809051136
There’s no official, native support for the <picture> element (or its polyfill) in WordPress, and the Picturefill.js script is not included in the WordPress distribution (like jQuery is), but you can use both the <picture> element along with Picturefill.js in your WordPress theme today, if you’re not afraid of a little custom theming. We’ll follow the instructions for using Picturefill.js by Scott Jehl for our actual <picture> element markup.