In CSS, canonical order is used to refer to the order in which separate values need to be specified (or parsed) or are to be serialized as part of a CSS property value. It is defined by the formal syntax of the property and normally refers to the order in which longhand values should be specified as part of a single shorthand value.
background shorthand property values are made up of several
background-* longhand properties. The canonical order of those longhand values is defined as
Furthermore, its syntax defines, that if a value for the
background-size is given, it must be specified after the value for the
background-position, separated by a slash. Other values may appear in any order.
- What does "canonical order" mean with respect to CSS properties? on Stack Overflow provides useful further discussion.
- The description of the formal syntax used for CSS values on MDN