The Myth of the Pixel Perfect Grid
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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 400px box viewed by a user browsing zoomed in is simply not 400px in CSS pixels. It may not have been 400px in device pixels even before they activated zoom.

See also: Ian Mallett’s Subpixel Zoo: A Catalog of Subpixel Geometry.