The picture element and associated features are W3C standard HTML features that allow web developers to deliver an appropriate image to every user depending on a variety of conditions like screen size, viewport size, screen resolution, and more.
Picturefill is a JavaScript file (or a polyfill to be more specific) that enables support for the picture element and associated features in browsers that do not yet support them, so you can start using them today!
Picturefill is developed and maintained by Filament Group, with help from the members of the Responsive Images Community Group and the developer community at large.
Picturefill