Picturefill

Markup

<img src="transparent.jpg" alt="..."
    sizes="(max-width: 40em) calc(100vw - 50px), 90vw"
    srcset="small.jpg 480w, medium.jpg 960w, large.jpg 1920w">
<noscript>
    <img src="small.jpg" alt="...">
</noscript>

Feature

<img> element & srcset attrubute (file width)