Use for normal img embeds. A good breakdown on image solutions: https://developers.google.com/web/fundamentals/design-and-ux/responsive/images
Polyfill: http://scottjehl.github.io/picturefill/ Demo: https://codepen.io/chrisjdesigner/pen/oOOWGv
<img src="400.png"
sizes="(min-width: 600px) 25vw, (min-width: 500px) 50vw, 100vw"
srcset="100.png 100w, 200.png 200w, 400.png 400w,
800.png 800w, 1600.png 1600w, 2000.png 2000w" alt="an example image">