• Proposal (Add a proposal to make room for plain-CSS functions and mixins #3812)
• Update official spec ([CSS Function/Mixin Prep] Mark as accepted and update spec #3854)
• Deprecation
  • Dart Sass (Deprecate function and mixin names beginning with -- dart-sass#2230)
  • Tests (Add deprecation tests for custom-ident functions and mixins sass-spec#1982)
  • Website (Document plain CSS function/mixin preparation sass-site#1028)
• CSS Compatibility (@function)
  • Spec (Parse plain CSS functions as unknown at-rules #4137)
  • Dart Sass (Add support for plain-CSS @function rules dart-sass#2655)
  • Tests (Add tests for plain-CSS @function sass-spec#2087)
  • Website (Document support for plain CSS functions sass-site#1494)
• CSS Compatibility (@mixin)
  • Spec
  • Dart Sass
  • Tests
  • Website

@mirisuzanne is working on a proposal for CSS custom functions and mixins at https://css.oddbird.net/sasslike/mixins-functions/

The suggested syntax uses @function and @mixin, which are the same at-rules than Sass, and so would clash with it right now.

The CSS proposal requires CSS custom functions and mixins to use dashed-ident, i.e. an identifier starting with --, to avoid clashes with built-in CSS functions (which never start with --).
However, Sass currently allows defining function and mixin names starting with --, as shown in https://sass-lang.com/playground/#eJwzNHRIK81LLsnMz1PQ1S1JLS5R0NBUqOZSUHAoSi0pLcpTUE9Wt+aq5eJyyM2syIQpMoKpSrFSSAVLJ4K5SVZQBRqa1iAzMvOSc0pTUmG6gCoBJ/UfhA==

Should we deprecate defining Sass members named using dashed-ident to reduce the risk of clashes with CSS ?