A Polymer implementation of the proposed <picture> element.
<x-picture alt="Description of content for accessability">
<source srcset="http://www.fillmurray.com/300/600 1x,
http://www.fillmurray.com/600/1200 2x" />
<source srcset="http://www.fillmurray.com/400/600 1x,
http://www.fillmurray.com/800/1200 2x"
media="(min-width:400px)" />
<source srcset="http://www.fillmurray.com/500/600 1x,
http://www.fillmurray.com/1000/1200 2x"
media="(min-width:500px)" />
<source srcset="http://www.fillmurray.com/600/600 1x,
http://www.fillmurray.com/1200/1200 2x"
media="(min-width:600px)" />
<source srcset="http://www.fillmurray.com/700/600 1x,
http://www.fillmurray.com/1400/1200 2x"
media="(min-width:700px)" />
<!-- This is a fallback for no javascript, not a fallback for no <picture /> support.
That's what the prollyfill does ;) -->
<noscript>
<img src="http://www.placecage.com/c/800/800" alt="Nic Cage" lazyload>
</noscript>
<p>Paradoxally, I am invisible AND accessible.</p>
</x-picture>