ranbena/index.html

## index.html
<html>
  <head>
    ...
    <link rel='stylesheet' href='retina.css' media='only screen and (min-device-pixel-ratio: 2)' />
    ...
  </head>
  <body>

## script.js
if (window.devicePixelRatio === 2) {
  // do something
}

## style.css
.logo {
  background-image: url('logo.png');
}

@media only screen and (min-device-pixel-ratio: 2) {
  .logo {
    background-image: url('logo@2x.png');
  }
}
	<html>
	<head>
	...
	<link rel='stylesheet' href='retina.css' media='only screen and (min-device-pixel-ratio: 2)' />
	...
	</head>
	<body>
	.logo {
	background-image: url('logo.png');
	}

	@media only screen and (min-device-pixel-ratio: 2) {
	.logo {
	background-image: url('logo@2x.png');
	}
	}