Canonical order
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.
For example, background shorthand property values are made up of several background-* longhand properties. The canonical order of those longhand values is defined as
background-imagebackground-positionbackground-sizebackground-repeatbackground-attachmentbackground-originbackground-clipbackground-color
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.
See also
- 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