The fact that we have to worry about an implementation detail like pixels at this point means something's broken. Not really. Apple might market their 300+ dpi

The fact that we have to worry about an implementation detail like pixels at this point means something's broken. Not really. Apple might market their 300+ dpi Retina displays on the basis that people can’t see the pixels any more, but in our experience working on a mobile-friendly site, that’s only true for some people and under some conditions. Meanwhile, the majority of web browsing is still done using devices with much lower pixel densities anyway, and on these devices extensive hinting and fine tuning can be necessary to get good results. We have a very long way to go before we can just describe an icon or font design in vector terms and have it appear correctly on every device. As supporting evidence, I observe that many web fonts look terrible on a typical desktop or laptop display, with quite a few being literally illegible on Windows XP. It’s trendy to blame that on Windows’ font rendering, and that is certainly a factor, but so is the reality that some of these fonts simply aren’t hinted and kerned very well at all. Just serving high-dpi assets isn’t a great fix, either. Aside from increasing the size of the download unnecessarily for users with relatively low-dpi screens, the scaling process often leaves awkward artifacts that wouldn’t have been there in, say, an icon that was designed from scratch to target that sort of pixel density. That’s partly because of hinting, and it’s partly because you can completely change the design (simplifying it, for example) if the larger/higher-dpi version is too complicated to work well. So for quite a while yet, we’re still going to have to produce and manually fine-tune assets at multiple sizes to get best results.