MiniCSS is a Ruby implementation of the CSS Syntax Level 3 tokenizer, parser, serializer, and selector toolkit. It provides fast, dependency-free primitives for building linters, minifiers, and other CSS-aware tooling.
- C tokenizer and parser for Ruby 3.4+.
- Battle-tested against the JSON-based
css-parsing-testssuite for standards-compliant parsing. - High-level AST wrappers (
MiniCSS::AST::Rule,Decl,AtRule,Function,Number, and more) ready for programmatic inspection. - Bidirectional selector helpers (
MiniCSS::Sel) for tokenizing, walking, computing specificity, and turning selector ASTs back into strings. - Serializer that converts AST nodes back into normalized CSS, enabling round-tripping and rewrites.
- Accepts either strings or IO objects, with optional Unicode range
tokenization via
allow_unicode_ranges.
Add MiniCSS to your project with Bundler:
bundle add minicssOr install it directly:
gem install minicssrequire "minicss"
css = <<~CSS
@media (min-width: 768px) {
.button.primary {
color: #fff;
background: rgb(0 0 0 / 80%);
}
}
CSS
sheet = MiniCSS.parse(css)
sheet.each do |node|
case node
when MiniCSS::AST::AtRule
puts "At-rule: @#{node.name} #{MiniCSS.serialize(node.prelude)}"
node.child_rules.each { puts " Child: #{MiniCSS.serialize(_1)}" }
when MiniCSS::AST::Rule
selector = MiniCSS::Sel.stringify(node.selector)
puts "Rule: #{selector}"
node.decls.each do |decl|
value = MiniCSS.serialize(decl.value)
important = decl.important? ? " !important" : ""
puts " #{decl.name}: #{value}#{important}"
end
when MiniCSS::AST::SyntaxError
warn "Syntax error: #{node.reason}"
end
endtokens = MiniCSS.tokenize(".btn { color: #fff; }")
tokens.map { |token| [token.kind, token.literal] }
# => [[:delim, "."], [:ident, "btn"], [:whitespace, " "], [:left_curly, "{"],
# [:whitespace, " "], [:ident, "color"], [:colon, ":"], [:whitespace, " "],
# [:hash, "#fff"], [:semicolon, ";"], [:whitespace, " "], [:right_curly, "}"]]
unicode_tokens = MiniCSS.tokenize("div { content: U+4E00-9FFF; }", allow_unicode_ranges: true)Every token carries positional information via token.pos_start and
token.pos_end, making it easy to surface diagnostics.
selector_ast = MiniCSS::Sel.parse("button#primary.action:hover")
MiniCSS::Sel.specificity(selector_ast)
# => [1, 2, 1]
MiniCSS::Sel.walk(selector_ast) do |token, parent|
puts "#{token[:type]} → #{token[:content]}"
end
# id → #primary
# class → .action
# pseudo-class → :hover
# type → button
MiniCSS::Sel.stringify(selector_ast)
# => "button#primary.action:hover"Selectors are represented as nested hashes and arrays, mirroring the structure
produced by Lea Verou’s parsel, which MiniCSS::Sel is based on.
MiniCSS.serialize accepts any AST node (or array of nodes) returned by
MiniCSS.parse:
ast = MiniCSS.parse(".card { margin: 1rem; padding: 1rem; }")
MiniCSS.serialize(ast)
# => ".card{margin:1rem;padding:1rem;}"This makes it easy to transform stylesheets and emit normalized output.
When the parser encounters invalid input it emits MiniCSS::AST::SyntaxError
nodes alongside the rest of the AST:
errors = MiniCSS.parse("p { color: }").grep(MiniCSS::AST::SyntaxError)
errors.each { warn _1.reason }
# Syntax error details, including source offsets, are preserved from the tokenizer.You can introspect the original token stream to report accurate diagnostics.
- Clone the repository, and run
git submodule update --initto fetch the required fixtures. - Install dependencies:
bundle install - Compile native extensions:
bundle exec rake compile - Run the test suite:
bundle exec rspec - Check style and linting:
bundle exec rubocop - Run everything:
bundle exec rake
Helpful utilities:
bin/consolestarts an IRB session with MiniCSS preloaded.bin/gen-specs.rbregenerates the specs underspec/parsing_testsfrom the externalcss-parsing-testsfixtures.benchmarks/ruby_prof.rbruns tokenization, parsing, and serialization against ruby-prof for spotting slow code.benchmarks/stylesheet.rbruns a benchmark for each parsing and serialization step against Crass.benchmarks/selectors.rbruns a benchmark for selector parsing, serialization, and specificity.
As MiniCSS now relies on C extensions for performance, comparisons below are not really fair, considering Crass is a pure-ruby solution. However, MiniCSS provides support for extra CSS features and a different way of handling ASTs.
Results below are running MiniCSS and Crass on the Bootstrap 4 CSS on both minified and unminified versions:
Warning
MiniCSS uses C extension for full tokenization. Comparisons below are not fair.
== Tokenize bootstrap-4.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS tokenize 3.000 i/100ms
Crass tokenize 1.000 i/100ms
Calculating -------------------------------------
MiniCSS tokenize 31.449 (± 3.2%) i/s (31.80 ms/i) - 159.000 in 5.058992s
Crass tokenize 12.731 (± 0.0%) i/s (78.55 ms/i) - 64.000 in 5.036126s
Comparison:
MiniCSS tokenize: 31.4 i/s
Crass tokenize: 12.7 i/s - 2.47x slower
== Tokenize bootstrap-4.min.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS tokenize 4.000 i/100ms
Crass tokenize 1.000 i/100ms
Calculating -------------------------------------
MiniCSS tokenize 39.820 (± 2.5%) i/s (25.11 ms/i) - 200.000 in 5.026530s
Crass tokenize 15.902 (± 6.3%) i/s (62.89 ms/i) - 80.000 in 5.036493s
Comparison:
MiniCSS tokenize: 39.8 i/s
Crass tokenize: 15.9 i/s - 2.50x slower
Note
MiniCSS uses a pure-ruby implementation for this section, except for the
TokenStream class, which relies on a C extension.
== Parse bootstrap-4.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS parse 1.000 i/100ms
Crass parse 1.000 i/100ms
Calculating -------------------------------------
MiniCSS parse 7.837 (± 0.0%) i/s (127.61 ms/i) - 40.000 in 5.105947s
Crass parse 8.554 (±11.7%) i/s (116.90 ms/i) - 43.000 in 5.061931s
Comparison:
Crass parse: 8.6 i/s
MiniCSS parse: 7.8 i/s - same-ish: difference falls within error
== Parse bootstrap-4.min.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS parse 1.000 i/100ms
Crass parse 1.000 i/100ms
Calculating -------------------------------------
MiniCSS parse 10.086 (± 0.0%) i/s (99.15 ms/i) - 51.000 in 5.058562s
Crass parse 11.333 (± 0.0%) i/s (88.24 ms/i) - 57.000 in 5.035570s
Comparison:
Crass parse: 11.3 i/s
MiniCSS parse: 10.1 i/s - 1.12x slower
Note
MiniCSS uses a pure-ruby implementaiton for this section. No C extensions are leveraged.
== Serialize bootstrap-4.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS serialize 26.000 i/100ms
Crass stringify 13.000 i/100ms
Calculating -------------------------------------
MiniCSS serialize 267.680 (± 1.1%) i/s (3.74 ms/i) - 1.352k in 5.051372s
Crass stringify 134.551 (± 3.0%) i/s (7.43 ms/i) - 676.000 in 5.028420s
Comparison:
MiniCSS serialize: 267.7 i/s
Crass stringify: 134.6 i/s - 1.99x slower
== Serialize bootstrap-4.min.css ==
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
MiniCSS serialize 26.000 i/100ms
Crass stringify 18.000 i/100ms
Calculating -------------------------------------
MiniCSS serialize 255.343 (± 5.1%) i/s (3.92 ms/i) - 1.300k in 5.105550s
Crass stringify 181.703 (± 5.0%) i/s (5.50 ms/i) - 918.000 in 5.065818s
Comparison:
MiniCSS serialize: 255.3 i/s
Crass stringify: 181.7 i/s - 1.41x slower
Bug reports, feature requests, and pull requests are welcome. By participating you agree to abide by the project’s Code of Conduct (see CODE_OF_CONDUCT.md). Please include tests whenever you add or change behavior.
The MIT License (MIT)
Copyright (c) 2025 Vito Sartori
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