An Au is an "App Unit" and represents 1/60th of a CSS pixel. It was originally proposed in 2002 as a standard unit of measure in Gecko. See https://bugzilla.mozilla.org/show_bug.cgi?id=177805 for more info.
An App Unit, the fundamental unit of length in Servo. Usually 1/60th of a pixel (see AU_PER_PX)
The number of app units in a pixel.