Drop support for Ruby Sass and LibSass

[36degrees]

What

Drop support for Ruby Sass and LibSass, including:

• removing mentions of them from the dependencies section of our install docs
• removing the tests for the Ruby Sass and LibSass compilers from our Sass CI workflow

Although there are no code changes, it's highly likely that we'll choose to make related breaking changes as part of this work. Regardless, it's a significant change for our users and so we should treat it as a breaking change.

Why

Ruby Sass was deprecated in April 2018 and reached End of Life in April 2019. We're not aware of any of our users still using it, but as supporting it has involved no extra work on our part we've supported it up until now.

LibSass has also been deprecated since October 2020.

The new Sass module system (@use) is only available in Dart Sass, and there are no plans to introduce it to LibSass. Additionally, a future release of Dart Sass will drop support for @import which will make it impractical for us to continue to support both Dart Sass and LibSass.

Dropping support for LibSass is also a recommendation made by the Sass maintainers themselves:

"We recommend all existing LibSass users make plans to eventually move onto Dart Sass, and that all Sass libraries make plans to eventually drop support for LibSass.

We’re no longer planning to add any new features to LibSass, including compatibility with new CSS features.

LibSass and Node Sass will continue to be maintained indefinitely on a best-effort basis, including fixing major bugs and security issues and maintaining compatibility with the latest Node versions."

Dropping support for Ruby Sass and LibSass will allow us to resolve the following issues:

• Migrate to Sass module system (@use) #1791
• Sass deprecation warning for slash as division (slash-div) #2238

Who needs to work on this

Developers, technical writer

Who needs to review this

Developers, technical writer

Done when

• Install docs make it clear that only Dart Sass is supported

[36degrees]

There's a presentation with more context on the changes going on in the eco-system, and some of the history of the three different compilers here: https://docs.google.com/presentation/d/10Jf6K-KsqhGyZIjcuxVL6P60ksJ4lSyT96WHZpJwARk/edit#slide=id.g10d42026b8_2_0

We've also started to communicate these upcoming changes to our users in our channels on x-gov Slack and as announcements in the GitHub Discussions for this repo.

[EoinShaughnessy]

Re the install docs update - are we removing all mention of LibSass and Ruby Sass, or are we just saying not to use them as we don't support them?

[36degrees]

I've added an entry to the 'Done when' to remind us to remove this check for LibSass:

govuk-frontend/packages/govuk-frontend/src/govuk/settings/_index.scss

Lines 23 to 34 in 45499f3

	// LibSass evaluates this as a single unquoted string, equivalent to
	// "true == false", whereas Dart Sass evaluates it as "true" == "false".
	// Because a string in Sass is truthy, LibSass will output the warning; Dart
	// Sass will not.
	@if #{true} == #{false} {
	@include _warning(
	"libsass",
	"It looks like you may be using LibSass to compile your Sass. LibSass is " +
	"deprecated and will not be supported by the next major version of " +
	"GOV.UK Frontend. See https://sass-lang.com/libsass/ for more information."
	);
	}