Img.srcset is a lightweight, no nonsense, all browser supporting, fast polyfill for img srcset, allowing for lighter yet backwards-compatible responsive web design.

the srcset attribute is an html extension for adaptive (a.k.a. responsive) images. more info at .

a polyfill is (in the context of html5) javascript code implementation of a functionality often available in modern web browsers, allowing web designers to use simpler standards-compliant and declarative code, burdening only older/simpler browsers with these fallback snippets.