Looks like CSS is getting extensibility built-in with a new @property element (works like @media – also looks like we're getting CSS.registerProperty() in JavaScript 🤔).

It'll work as below, and let you extend existing CSS functions with a similar syntax to CSS variables. Not 100% sure about how useful it will be, but it's always nice to see added flexibility and certainly the gradient animation use case provided in the example would have its purposes.

@property --my-property {
    syntax: '<color>';
    initial-value: green;
    inherits: false;
}