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A cross-implementation Sass test suite

Build Status

sass-spec is the official Sass test suite. It's used by all major Sass implementations to ensure that they correctly implement the language.

Running Specs

Before running specs, you'll need to install Ruby and Bundler. The spec runner is written in Ruby, so it's necessary no matter which implementation you're testing. Then, from the root of this repo, run bundle install.

From there, it depends which implementation you're testing:

Dart Sass

To run specs against Dart Sass, the reference implementation of Sass that's used for the sass package on npm, you'll first need to install Dart. Then run:

# If you already have a clone of the Dart Sass repo, you can use that instead.
git clone
(cd dart-sass; pub get)
export DART_SASS_PATH=`pwd`/dart-sass

bundle exec ./sass-spec.rb --dart $DART_SASS_PATH


To run specs against LibSass, the C++ Sass implementation that's used for Node Sass and other languages' Sass wrappers, you'll need to be able to build LibSass. Once you have all the build dependencies:

# If you already have a clone of the LibSass repo, you can use that instead.
git clone
(cd libsass; ./script/bootstrap; make sassc)
export SASSC_PATH=`pwd`/libsass/sassc/bin/sassc

bundle exec ./sass-spec.rb --impl libsass -c $SASSC_PATH

Spec Structure

Each spec is defined by a directory with an input.scss or input.sass file and either:

  • An output.css file, in which case the spec asserts that the Sass implementation compiles the input to the output. These specs are known as "success specs".
  • An error file, in which case the spec asserts that the Sass implementation prints the error message to standard error and exits with a non-zero status code when it compiles the input. These specs are known as "error specs".

These files may also have variants that are specific to individual implementations.

The path to the spec serves as the spec's name, which should tersely describe what it's testing. Additional explanation, if necessary, is included in a silent comment in the input file. Specs may also contain additional files that are used by the input file, as well as various other features which are detailed below.


Most specs are stored in HRX files, which are human-readable plain-text archives that define a virtual filesystem. This format makes it easy for code reviewers to see the context of specs they're reviewing. The spec runner treats each HRX file as a directory with the same name as the file, minus .hrx. For example:

<===> input.scss
ul {
  margin-left: 1em;
  li {
    list-style-type: none;

<===> output.css
ul {
  margin-left: 1em;
ul li {
  list-style-type: none;

HRX archives can also contain directories. This allows us to write multiple specs for the same feature in a single file rather than spreading them out across hundreds of separate tiny files. By convention, we include an HRX comment with 80 = characters between each spec to help keep them visually separate. For example:

<===> unbracketed/input.scss
a {b: is-bracketed(foo bar)}

<===> unbracketed/output.scss
a {b: false}

<===> bracketed/input.scss
a {b: is-bracketed([foo bar])}

<===> bracketed/output.scss
a {b: true}

Each HRX archive shouldn't be much longer than 500 lines. Once one gets too long, its subdirectories should be split out into separate archives beneath a physical directory. Conversely, if a given directory contains many small HRX archives, they should be merged together into one larger file. This helps ensure that the repo remains easy to navigate.

The only specs that aren't written in HRX format are those that include invalid UTF-8 byte sequences. The HRX format is itself written in UTF-8, so it's unable to represent the files in these specs.

Specifying Warnings

By default, Sass implementations are expected to emit nothing on standard error when executing a success spec. However, if a warning file is added to the spec directory, the spec will assert that the Sass implementation prints that warning message to standard error as well as compiling the output. This is used to test the behavior of the @debug and @warn rules, as well as various warnings (particularly deprecation warnings) emitted by the Sass implementation itself.

Warnings can't be specified for error specs, since everything an implementation emits on standard error is considered part of the error message that's validated against error.

Implementation-Specific Expectations

Sometimes different Sass implementations produce different but equally-valid CSS outputs or error messages for the same input. To accommodate this, implementation-specific output, error, and warning files may be created by adding -dart-sass or -libsass after the file's name (but before its extension, in the case of output.css).

When a spec is running for an implementation with an implementations-specific expectation, the normal expectation is ignored completely in favor of the implementation-specific one. It's even possible (although rare) for one implementation to expect an input file to produce an error while another expects it to compile successfully.


Metadata for a spec and options for how it's run can be written in an options.yml file in the spec's directory. This file applies recursively to all specs within its directory, so it can be used to configure many specs at once. All options must begin with :.

All options that are supported for new specs are listed below. A few additional legacy options exist that are no longer considered good style and will eventually be removed.


- sass/libsass#2827

This option indicates implementations that should add support for a spec, but haven't done so yet. When running specs for a given implementation, all specs marked as :todo for that implementation are skipped by default. This ensures that the build remains green while clearly marking which specs are expected to pass eventually.

Implementations can be (and should be) specified as shorthand GitHub issue references rather than plain names. This makes it easy to track whether the implementation has fixed the issue, and to see which specs correspond to which issue. When marking an issue as :todo for an implementation, please either find an existing issue to reference or file a new one.

If the --run-todo flag is passed to sass-spec.rb, specs marked as :todo for the current implementation will be run, and their failures will be reported.

If the --probe-todo flag is passed to sass-spec.rb, specs marked as :todo for the current implementation will be run, but a failure will be reported only if those specs pass. This is used to determine which specs need to have :todo removed once a feature has been implemented. This can be used in combination with --interactive to automatically remove :todos for these specs.


- sass/libsass#2834

This option works like :todo, except instead of skipping the entire test for listed implementations it only skips validating that spec's warnings. The rest of the spec is run and verified as normal. This should not be used for error specs.


- libsass

This option indicates implementations that are never expected to be compatible with a given spec. It's used for specs for old features that some but not all implementations have dropped support for.

Spec Style

The specs in this repo accumulated haphazardly over the years from contributions from many different people, so there's not currently much by way of unified style or organization. However, all new specs should follow the style guide, and old specs should be migrated to be style-guide compliant whenever possible.

Interactive Mode

If you pass --interactive to sass-spec.rb, it will run in interactive mode. In this mode, whenever a spec would fail, the spec runner stops and provides the user with a prompt that allows them to inspect the failure and determine how to handle it. This makes it easy to add implementation-specific expectations or mark specs as :todo. For example:

In test case: spec/core_functions/color/hsla/four_args/alpha_percent
Output does not match expectation.
i. Show me the input.
d. show diff.
O. Update expected output and pass test.
I. Migrate copy of test to pass on libsass.
T. Mark spec as todo for libsass.
G. Ignore test for libsass FOREVER.
f. Mark as failed.
X. Exit testing.

Any option can also be applied to all future occurences of that type of failure by adding ! after it. For example, if you want to mark all failing specs as :todo for the current implementation you'd type I!.


The unit tests for the spec runner are located in the tests/ directory. To run these unit tests, run:

bundle exec rspec tests/
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