W3C compliant optimization for retina display images @2x.png

How to insert a picture 2x retina display with W3C compliant rules,
the image is displayed with the size of the css to 50% of actual size.

// HTML
<img src="/[email protected]" alt="logo" id="PNGlogo" width="200" height="200">

// CSS
#PNGlogo {
	width:100px;
	height:100px;
}