Preparing for Plain CSS Functions and Mixins: Draft 1

(Issue)

This proposal adds errors for existing Sass syntax which is likely to conflict with upcoming plain CSS support for functions and mixins.

Table of Contents

• Background
• Summary
• Semantics
  • @mixin
  • @function
• Deprecation Process

Background

This section is non-normative.

In this informal proposal and this issue, the CSS working group have begun seriously discussing the possibility of introducing functions and mixins to CSS itself. Although many specific details are still in flux, all syntaxes that have been floated have two things in common:

• They use at-rules named @mixin and @function, just as Sass does today.
• They require custom identifiers beginning with -- after these at-rules.

Sass currently allows functions and mixins whose names begin with --, but this is a relatively painless thing to deprecate and would allow us full CSS compatibility with whatever version of the proposal ends up getting implemented.

Summary

This section is non-normative.

This proposal makes any mixin or function whose name begins with -- an error. This will be preceded by a deprecation period in which those mixins and functions will just produce warnings.

Semantics

@mixin

Update the @mixin semantics to add after the first line:

• If name begins with --, throw an error.

@function

Update the @function semantics to add after the first line:

• If name begins with --, throw an error.

Deprecation Process

During the deprecation period, instead of throwing errors as described in the semantics section above, emit a deprecation warning named css-function-mixin.