This is only showing a potential polyfill for
srcset attribute on img. It's only a quick proof of concept.
The minimum (and useful) code that should have a reasonable fallback story is:
<noscript><img src="small.png" alt="Alt here"></noscript>
alt on the img in
style for Internet Explorer, it shows a broken image icon without (you may be okay with that, and can remove style if so).