Breaking Change: Functions and Mixins Beginning with 

Prior to Dart Sass 1.76.0, function and mixin names could be any valid CSS identifier, but identifiers beginning with -- are now deprecated.

Generally, Sass allows any valid CSS identifier to be used for any Sass definition. This includes identifiers which begin with --, which users may be most familiar with in the context of CSS custom properties. However, the CSS working group is seriously considering adding built-in support to CSS itself for functions and mixins, likely using at-rules named @mixin and @function just like Sass.

This means that Sass, in order to preserve its core design principle of CSS compatibility while still supporting Sass’s build-time functions and mixins, needs to be able to distinguish between CSS and Sass declarations that use the same at-rule names. Fortunately, although the details of the syntax CSS uses for functions and mixins is still very much up in the air, one point seems uncontroversial: the use of custom-property-like identifiers beginning with -- for CSS mixin and function names.

This will allow Sass to distinguish plain-CSS functions and mixins as those that begin with --. But in order for this to work, we first have to disallow Sass functions and mixins from using that prefix.

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Dart Sass
since 1.76.0
Ruby Sass

Between Dart Sass 1.76.0 and Dart Sass 2.0.0, Dart Sass will continue to allow functions and mixins whose names begin with --. However, it will emit a deprecation warning named css-function-mixin.

Between Dart Sass 2.0.0 and the release of Dart Sass’s support for plain CSS functions and mixins, functions and mixins will not be allowed to have names that begin with --.