To make images on the ipad3 look great, all we have to do is change:
<img src="photo.jpg" width=800 height=600 />
to:
<img src="photo.jpg" srcset="photo@2x.jpg 1600w 1200h" width=800 height=600 />
That is a ton cleaner than dealing with <picture> which completely breaks most browsers.
One minor point, however, is that the strawman <picture> proposal was meant to be equally backward compatible through the use of a child <img> tag that would only display in unsupported browsers, as <video> works today:
<picture alt="cool-image">
<source src=hires.png media="max-width:1920px">
<source src=midres.png media="min-width:800px">
<source src=lores.png>
<!-- fallback for browsers without support -->
<img src=midres.png alt="cool-image">Also, <video> fallback doesn't really work. If you have an h264 video, there is no good way to make it play in all browsers with markup alone (Firefox won't fallback to flash).