Code:

<img alt="@srcset experiments"
	src="srcset_base.gif"
	srcset="srcset_2x.gif 2x,
		srcset_768w.gif 768w,
		srcset_768w_2x.gif 768w 2x,
		srcset_1024w.gif 1024w,
		srcset_1024w_2x.gif 1024w 2x"
	style="display:block;margin:1em auto;border:2px solid #000;">

Alternate code, verify browser attributes:

http://www.webkit.org/demos/srcset/
http://atmedia.info