gistfile1.txt

<picture>
	<!-- 16:9 crop -->
	<source
		media="(min-width: 40em)"
		srcset=“images/wide/large.jpg  1920w,
		        images/wide/medium.jpg  960w,
		        images/wide/small.jpg   480w" />
		sizes=“100vw"

	<!-- square crop -->
	<source
		srcset="images/square/large.jpg  822w,
		        images/square/medium.jpg 640w,
		        images/square/small.jpg  320w" />
		sizes=“100vw"
	<img
		src="images/wide/medium.jpg"
		alt="Image description text." />
</picture>