"In" stands for CSS inches — an absolute unit, where 1in=96px. On high-resolution devices like laser printers today, a CSS inch will be the same size as its physical dimension, and that's when inches and other absolute units are mostly used.

But how often is CSS used for creating printed media? Actually, quite often. A common use case would be when a webpage has a separate style sheet for its version for printing to make sure it's legible, and the layout doesn't change. CSS is also used to format books, catalogs, and brochures — content that may never have been designed to be a web page at all.