Retina display Macs are only going to be a small percentage of the PC surfing audience for now, but Apple tends to lead the market on tech specs. Sony are already building devices with a 'Reality Display' at 233 DPI, and others are likely to follow.
The point I was trying to get at is that you need bandwidth detection as well as browser detection. A 300DPI monitor on a 3G connection needs a combination of responsive design and low-bandwidth images, not just the super-duper high-resolution page. You can't assume that a high-resolution screen is going to be connected to a fat data pipe.
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