valid value definition
Value definition syntax. A formal grammar, the CSS value definition syntax, is used for defining the set of valid values for a CSS property or function. In addition to this syntax, the set of valid values can be further restricted by semantic constraints (like, for a number to be strictly positive). The definition syntax describes which values are allowed and the interactions between them. A component can be a keyword, some characters considered as a literal, or a value of a given CSS data type or of another CSS property.