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?

Using srcset-polyfill to reduce bandwidth for mobile devices

If you are wanting to serve smaller images to mobile devices to reduce bandwidth it is important to set your syntax correctly to avoid downloading the mobile optimised image and the original (larger) image. The correct syntax to use is:

<img src="small.jpg" srcset="small.jpg 320w, medium.jpg 960w, large.jpg" />

#### Notes

• Include the smallest image in the src attribute (in the above example: small.jpg).
• Include the smallest image and its associated max viewport width in the srcset attribute (in the above example: small.jpg 320w).
• Include any other, wider viewport widths in the srcset attribute (in the above example: medium.jpg 960w).
• Include the full size image in the srcset attribute, without any viewport width restriction (in the above example; large.jpg).