A spec-compatible, unit-tested polyfill for <img srcset>

See the specification for the reference algorithm.

Usage

Use the srcset attribute of <img> elements. For example:

<img alt="The Breakfast Combo"
     src="banner.jpeg"
     srcset="banner-HD.jpeg 2x, banner-phone.jpeg 100w,
             banner-phone-HD.jpeg 100w 2x"/>

Include build/srcset.min.js in your page.

Open questions

• How to reliably check for srcset support in the browser (so as to not attempt to polyfill if it's not necessary?)
• Is it safe to use -webkit-transform to scale things?
• Is it worth falling back to -webkit-image-set if available?