Created attachment 357040 [details]
Safari 12 HiDPI screenshot
The both text on the previous example looks fine on iPhones, iPads,normal DPI Safari, Chrome and Firefox. The problem is that the top text is blurry on HiDPI Safari (retina iMac for example)
To reproduce the issue on normal DPI screen:
sudo defaults write /Library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool true
Then System Preferences -> Display and choose the HiDPI at half resolution: https://cdn1.tekrevue.com/wp-content/uploads/2015/08/hidpi-mode-system-preferences.png
I think this is a quirk of Core Animation; we've fixed this for iOS in bug 169457, but I think we need the same fix for macOS.