CSS custom functions and mixins

The CSS custom functions and mixins module allows developers to create reusable blocks of CSS code that can accept arguments, contain complex logic (defined using features such as CSS if() functions and @media at-rules), and return values based on that logic.

CSS custom functions are defined in @function at-rules and called using <dashed-function> syntax, which looks very similar to CSS custom property syntax except that it includes parentheses at the end containing arguments (for example, --my-function(30px, 3)). CSS custom functions can be called within any property value, and will return a value based on the arguments passed into the function and the logic inside it.

CSS mixins are defined in @mixin at-rules, and applied using @apply at-rules nested inside rulesets. CSS mixins define a set of properties, which can be reused inside multiple rulesets, as well as being customized with arguments and logic.

CSS custom functions and mixins can be assigned optional data types for their arguments and return values, to strictly control the values passed into them and returned from them.