CSS Pixel

One of the units that we can use in CSS is a px. The name is confusing because the CSS px does not represent the display pixel. Instead, it is a Reference Pixel. The general rule is that 96 px (CSS px) = 1 inch. This is physically true on hi-dpi displays, and less true on low-dpi ones. W3C initially required all CSS engines to render absolute sizes correctly, according to physical dimensions. However, many implementations did not follow that and W3C eventually dropped that requirement (source).

It is important because it means that our current web pages that have sizes defined in px will continue to work in the future as higher-resolution screens are developed.