sass/CONTRIBUTING.md

When we add a new feature to Sass, we want to make sure the feature is
well-designed, clearly specified, feasible to implement, and that it meets the
use-cases it's designed for.

## Process

The process for adding a new feature works as follows:

1. The feature is informally discussed on [the issue tracker][]. Most new
   features come directly from use-cases brought up by Sass users, or new CSS
   syntax that Sass needs to support. Once the Sass team has agreed that a
   feature is desirable, it's marked as [Planned][] and can move to step 2.

   [the issue tracker]: https://github.com/sass/sass/issues
   [Planned]: https://github.com/sass/sass/labels/Planned

2. A formal proposal is written for the feature, following the format [outlined
   below](#proposal). This proposal is sent as a pull request, where the Sass
   team will discuss its specifics with the author. If/when everyone agrees on a
   first draft, the pull request will be accepted and the feature moves to step
   3.

   Step 2 is also where issues are opened for each individual implementation to
   add the feature. These issues should link to the feature's main issue in the
   [sass/sass][] issue tracker, and that issue should link back to the
   implementation issues.

   [sass/sass]: https://github.com/sass/sass

3. Public comments are solicited for the feature, usually via a tweet from
   [@SassCSS][]. If the feature is big enough, a blog post soliciting feedback
   may also be written. Then we await comments and iterate on feedback for an
   amount of time that varies based on the size of the feature and the amount of
   feedback received.

   [@SassCSS]: https://twitter.com/SassCSS
   
   As the proposal is updated based on feedback, its draft number should be
   increased according to the [versioning policy][] and changes should be logged
   in a changelog file named `<proposal>.changes.md`. Once enough time has
   elapsed and the Sass team is satisfied that all feedback is addressed, the
   feature moves to step 4.

   [versioning policy]: https://github.com/sass/language#versioning-policy

4. The proposal is marked as accepted and moved into [the `accepted/`
   directory][]. *This doesn't mean that the proposal is immutable*, but it does
   mean that no major changes to its semantics are expected. At this point, it's
   time to write [specs][] for the new feature, in tandem with implementing it
   in [Dart Sass][] (since it's the reference implementation). Writing the specs
   alongside an implementation helps ensure that the specs are accurate and
   sensible, and that the implementation is correct.
   
   [the `accepted/` directory]: https://github.com/sass/language/tree/master/accepted
   [specs]: https://github.com/sass/sass-spec
   [Dart Sass]: https://github.com/sass/dart-sass

   The new specs should have an `options.yml` file that marks them as TODO for
   LibSass, with a reference to its issue for the new feature, and marks them as
   ignored for Ruby Sass. For example:
   
   ```yaml
   ---
   :todo:
   - libsass # sass/libsass#2701
   :ignore_for:
   - ruby-sass
   ```

   Once the specs and the implementation are complete, they're sent as pull
   requests to [sass-spec][] and [Dart Sass][], respectively. They need to have
   special lines in their pull request messages in order to build properly:
   
   [sass-spec]: https://github.com/sass/sass-spec

   * The sass-spec pull request message should include `[skip dart-sass]`. This
     will cause it not to run Dart Sass tests, which would otherwise fail
     because the implementation of the new feature hasn't landed yet.

   * The Dart Sass pull request's message should link to the sass-spec pull
     request (for example, `See sass/sass-spec#1293`). This will cause it to run
     against the specs in that pull request and so test your new feature.

   Once these pull requests land, the feature moves to step 5.

5. The feature is eventually implemented in LibSass. Once this happens, the
   original issue can be closed, and the feature is considered fully
   implemented.

## Proposal

A good feature proposal should make it possible for an average Sass user to
understand and discuss the feature and the context around it, and possible for
Sass maintainers to implement consistent and well-defined behavior. The
following outline is designed to make satisfy these needs.

A proposal must include at minimum a Summary and a Syntax *or* a Semantics
section. Everything else is optional. Proposals may include additional sections,
or divide a section into sub-sections, as necessary to make it clear and
readable. All proposals should also include tables of contents that link to all
their sections.

Everything in sections that aren't explicitly marked as non-normative should be
construed as part of the specification of the feature. Non-normative notes can
be included inline in normative sections using [blockquotes][].

[blockquotes]: https://daringfireball.net/projects/markdown/syntax#blockquote

See [the `accepted/` directory][] for examples of proposals that have been
accepted.

* **Background**

  This non-normative section describes the broader context for the feature. This
  is particularly relevant for changes to existing syntax, and *especially* for
  backwards-incompatible changes. It should explain Sass's current behavior, the
  original reasoning behind that behavior, and why it's insufficient.

  See [Plain CSS `min()` and `max()`][min-max background] for a good example of
  a Background section.

  [min-max background]: https://github.com/sass/language/blob/master/accepted/min-max.md#background

* **Summary**

  This non-normative section provides a concise, user-friendly summary of the
  behavior being proposed. It doesn't need to be fully explicit about every
  corner of the feature, it just needs to give users an idea of how it works and
  what use-cases it addresses. Code examples are encouraged.
  
  See [Escapes in Identifiers][] for a good example of a Summary section.

  [Escapes in Identifiers]: https://github.com/sass/language/blob/master/accepted/identifier-escapes.md#summary

  * **Design Decisions**
  
    This sub-section goes into detail about decisions that were made during the
    design of the feature. It should describe alternatives that were considered,
    and explain why the final decision was made the way it was.

    See [Plain CSS `min()` and `max()`][min-max design] for a good example
    of a Design Decisions section.
    
    [min-max design]: https://github.com/sass/language/blob/master/accepted/min-max.md#design-decisions

* **Syntax**

  This section describes the syntax of the feature being added, if it adds new
  syntax to the language. The syntax should be written in [Backus-Naur form][],
  with regular expression-style operators and the convention that nonterminals
  are written in capitalized camel-case form. For example:

  <x><pre>
  **MinMaxExpression** ::= CssMinMax | FunctionExpression
  **CssMinMax**        ::= ('min(' | 'max(') CalcValue (',' CalcValue)* ')'
  **CalcValue**        ::= CalcValue (('+' | '-' | '*' | '/') CalcValue)+
  &#32;                  | '(' CalcValue ')'
  &#32;                  | ('calc(' | 'env(' | 'var(') InterpolatedDeclarationValue ')'
  &#32;                  | CssMinMax
  &#32;                  | Interpolation
  &#32;                  | Number
  </pre></x>

  [Backus-Naur form]: https://en.wikipedia.org/wiki/Backus–Naur_form

  Syntax definitions can also refer to productions from CSS specs. The proposal
  should link to the specs in question.

  See [Range-Context Media Features][] for an good example of a Syntax section.

  [Range-Context Media Features]: https://github.com/sass/language/blob/master/accepted/media-ranges.md

* **Semantics**

  This section describes the runtime behavior of the new feature. It may be
  omitted if the feature only has to do with how the stylesheet is parsed. The
  semantics section covers everything about how a stylesheet is evaluated,
  including how imports are resolved and the behavior of built-in functions.

  See [CSS Imports][css-imports semantics] for a good example of a Semantics section.

  [css-imports semantics]: https://github.com/sass/language/blob/master/accepted/css-imports.md#semantics

* **Deprecation Process**

  All backwards-incompatible features should go through a deprecation process if
  at all possible (see [Dart Sass's compatibility policy][]). This section
  describes the details of that process, including what code will produce
  deprecation warnings and how those warnings will indicate what the user should
  do to make their stylesheet forwards-compatible.

  [Dart Sass's compatibility policy]: https://github.com/sass/dart-sass#compatibility-policy

  See [CSS Imports][css-imports deprecation] for a good example of a Deprecation
  Process section.

  [css-imports deprecation]: https://github.com/sass/dart-sass#compatibility-policy