Don't know where you get the idea of an extra request from. What they have implemented is the srcset attribute (from the WHATWG HTML spec), which means that authors can write e.g.
<img alt="Slashdot" src="slashdot_logo.png" srcset="slashdot_logo_big.png 2x">
and the browser will then choose based on the viewport size and resolution which URL to load (whereas browsers that don't support this attribute will just load what's in src). In this case, it would load the big image if the resolution was at least 2*96dpi.