Terence Eden explains how different screen technologies, human biology, and fingerprint grease make “pixel perfection” a pointless goal:
There is no grid. There never has been. You can align to theoretical pixels - but as soon as the image hits a physical screen, it will be adjusted to best fit reality.
An obsession with pixel perfect rendering is futile.
Every Layout expands on this idea, specifically as it pertains to CSS:
Suffice it to say that, while screens are indeed made up of pixels, pixels are not regular, immutable, or constant. A
400pxbox viewed by a user browsing zoomed in is simply not
400pxin CSS pixels. It may not have been
400pxin device pixels even before they activated zoom.
See also: Ian Mallett’s Subpixel Zoo: A Catalog of Subpixel Geometry.