WebKit now supports the srcset attribute, making it easier to deliver high resolution images to high resolution displays without inconveniencing anyone else.
The srcset attribute is similar to the -webkit-image-set CSS function, in that it uses 1x, 2x, 3x modifiers to tell the browser which image to choose. For example, the 2x modifier marks the image that should be used if the display has more than two device pixels per CSS pixel. A blog post from WebKit.org explains that the srcset attribute can be thought of as "the markup equivalent to the CSS feature".
It is backwards compatible, so browsers that don't support the attribute will ignore it and fall back the src value.
You can read more about it on the official specification page at W3C.