accent-sass

Sass infrastructure for Rust, at parity with dart-sass.

accent-sass compiles Sass to CSS in pure Rust, with no Node and no libsass. It is a fork of connorskees/grass that carries the modern Dart Sass features upstream has not released, so that a Rust program can build real-world stylesheets -- Bulma, Pico, Foundation, USWDS -- without shelling out to another toolchain.

Parity with dart-sass is the goal, not an aspiration to approximate it: a deviation from the reference implementation is a bug, except in error messages and error spans.

It is built for and maintained alongside Accent CMS, the single-binary markdown CMS, which compiles theme Sass in-process through this crate. It is a general-purpose library, and does not depend on Accent.

Documentation: https://zoosky.github.io/accent-sass -- a usage guide, a reference, and a demo that compiles Bulma and USWDS from source in your browser. The site is built from docs/ with Accent CMS, which compiles its stylesheet with this compiler.

Install

accent-sass = "0.16.0"

To track work that has not been released yet, pin a git revision instead:

accent-sass = { git = "https://github.com/zoosky/accent-sass.git", rev = "<commit>" }

Use

As a library:

fn main() -> Result<(), Box<accent_sass::Error>> {
    let css = accent_sass::from_string(
        "a { b { color: &; } }".to_owned(),
        &accent_sass::Options::default(),
    )?;
    assert_eq!(css, "a b {\n  color: a b;\n}\n");
    Ok(())
}

The API is deliberately small: from_string, from_path, from_string_with_file_name, and an Options builder. Options::fs takes any Fs implementation, so a host that already holds its stylesheets can compile them without writing them to disk first -- MemoryFs is one such implementation, and is what the browser build resolves imports through.

As a binary, intended as a drop-in for the sass executable:

accent-sass input.scss                # compile to stdout
accent-sass --check app.scss app.css  # verify app.css is up to date

--check compiles and writes nothing, exiting 3 when the output file is stale or missing and 1 when the stylesheet does not compile, so a CI job can tell "your CSS is out of date" from "your Sass is broken".

As a browser package, built with wasm-pack:

import init, { compileString } from "accent-sass";

await init();

const { css, loadedUrls } = compileString('@use "theme";', {
  files: {
    "theme/_colors.scss": "$brand: #bada55 !default;",
    "theme/_index.scss": '@forward "colors";',
  },
  style: "expanded",
  logger: (event) => console.warn(event.message),
});

Every file a compile might touch must be in files before you call it. Reads are synchronous: the compiler asks for a file and gets bytes back, with nothing to await, so an importer cannot fetch, cannot await, and cannot reach the File System Access API. An editor loads the theme's stylesheets into the map first, then compiles. Making imports async would mean an async evaluator, which is a rewrite rather than a binding change.

Options are named after dart-sass's JavaScript API wherever the two have the same knob: style, syntax, loadPaths, charset, alertAscii and url, plus files, logger, and quiet for the one knob dart-sass has no name for. A failed compile throws an Error carrying message, formatted, file, line and column.

Build it with:

wasm-pack build crates/lib --release --target web --out-name index -- \
  --no-default-features --features wasm-exports,random

wasm-exports is not a default feature. Without it wasm-bindgen exports nothing and the module contains no compiler at all. docs/demo is a page built on it, compiling Bulma and USWDS from source in the browser.

Status

14,147 of 14,266 sass-spec tests pass against the pinned spec revision b39c32768, leaving 111 failures (measured on macOS 2026-09-13; CI's Linux runner reports two fewer passing). CI compiles Bulma, Pico, Foundation and USWDS with both engines on every commit and fails on any colour-value difference; all four currently compile byte-identically to dart-sass.

What each release changed is in CHANGELOG.md. What is left is in specs/docs/features/, one document per work item, ranked by the spec tests it unlocks.

accent-sass is not a drop-in replacement for libsass and does not intend to be.

Cargo features

Testing

Running cargo test should be all you need. The crate keeps a suite distinct from sass-spec, following the same philosophy as rust-analyzer, so tests run without ruby and can be more granular than the official spec.

To run the official suite (node >= v14.14.0; does not work on Windows):

git clone https://github.com/zoosky/accent-sass --recursive
cd accent-sass && cargo build --release
cd sass-spec && npm install
npm run sass-spec -- --impl=dart-sass --command '../target/release/accent-sass' \
  --trim-errors --ignore-warning-diffs --ignore-error-diffs

The leniency flags score CSS output and error messages only. Without them a test that differs solely in a missing deprecation warning or in error wording counts as a failure; that gap is sized in specs/docs/features/08-calculation-warnings-and-error-wording.md.

Versioning

Semantic Versioning. While the major version is 0, a breaking change bumps the minor version. Version numbers are this fork's own and do not track upstream grass.

The crates are on the Rust 2024 edition. The minimum supported Rust version is 1.96.1, normalised across the Accent crates; CI gates on it. Raising the MSRV is a minor version bump.

accent-sass targets dart-sass version 1.104.1.