Add a proposal to make room for plain-CSS functions and mixins

See #3787

proposal/css-function-mixin-preparation.md

@@ -0,0 +1,67 @@
+# Preparing for Plain CSS Functions and Mixins: Draft 1
+
+*([Issue](https://github.com/sass/sass/issues/3787))*
+
+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](#background)
+* [Summary](#summary)
+* [Semantics](#semantics)
+  * [`@mixin`](#mixin)
+  * [`@function`](#function)
+* [Deprecation Process](#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:
+
+[this informal proposal]: https://css.oddbird.net/sasslike/mixins-functions/
+[this issue]: https://github.com/w3c/csswg-drafts/issues/9350
+
+* 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:
+
+[the `@mixin` semantics]: ../spec/at-rules/mixin.md
+
+* If `name` begins with `--`, throw an error.
+
+### `@function`
+
+Update [the `@function` semantics] to add after the first line:
+
+[the `@function` semantics]: ../spec/at-rules/function.md
+
+* 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`.
+
+[the semantics section]: #semantics