Once again, I find myself surprised by someone's lack of surprise. Do you really think Hacker News uses hi-dpi images? My mind would be blown if it turned out it did. The HTML here is very old-school.
Note also, that the zooming that is done on mobile browsers, and (as I understand it on most modes for the retina MBP) doesn't simply multiply by 2 like native apps on the iPad/Phone so even if you provided a double resolution image then it would be unlikely for it to be displayed "pixel perfect", but the increased resolution (in image and screen) would help to hide any resizing artefacts better. That's why using Unicode characters is a better idea, when possible, as they are vector and scale nicely, particularly so on high-dpi devices.
Note also, that the zooming that is done on mobile browsers, and (as I understand it on most modes for the retina MBP) doesn't simply multiply by 2 like native apps on the iPad/Phone so even if you provided a double resolution image then it would be unlikely for it to be displayed "pixel perfect", but the increased resolution (in image and screen) would help to hide any resizing artefacts better. That's why using Unicode characters is a better idea, when possible, as they are vector and scale nicely, particularly so on high-dpi devices.