CSS processors

The linter supports current and future CSS syntax. This includes all standard CSS but also special features that use standard CSS syntactic structures, e.g. special at-rules, special properties, and special functions. Some CSS-like language extensions -- features that use non-standard syntactic structures -- are, as such, supported; however, since there are infinite processing possibilities, the linter cannot support everything.

You can run the linter before or after your CSS processors. Depending on which processors you use, each approach has caveats:

Before: Some plugins/processors might enable a syntax that isn't compatible with the linter.
After: Some plugins/processors might generate CSS that is invalid against your linter config, causing violations that do not correspond to your original stylesheets.

In both cases you can either turn off the incompatible linter rule, or stop using the incompatible plugin/processor. You could also approach plugin/processor authors and request alternate formatting options that will make their plugin/processor compatible with stylelint.

Parsing non-standard syntax

stylelint will automatically infer the syntax from the:

file extension
type or lang attribute on <style> tags in HTML (and HTML-like)
info string on GFM fenced code blocks in Markdown

You can force a specific syntax, though. Both the CLI and the Node.js API expose a syntax option.

If you're using the CLI, use the syntax flag like so: stylelint ... --syntax scss.
If you're using the Node.js API, pass in the syntax option like so: stylelint.lint({ syntax: "sugarss", ... }).

stylelint can also accept a custom PostCSS-compatible syntax when using the CLI or Node.js API. For custom syntaxes, use the custom-syntax and customSyntax options, respectively.

If you're using the CLI, use the custom-syntax flag like so: stylelint ... --custom-syntax custom-syntax-module or stylelint ... --custom-syntax ./path/to/custom-syntax-module.
If you're using the Node.js API, pass in the customSyntax option like so: stylelint.lint({ customSyntax: path.join(process.cwd(), './path/to/custom-syntax-module') , ... }).

If you‘re using the linter as a PostCSS Plugin, you should use the special postcss-syntax directly with PostCSS’s syntax option like so:

var postcss = require("postcss")
var syntax = require("postcss-syntax")

postcss([
  require("stylelint"),
  require("reporter")
])
  .process(css, {
    from: "lib/app.css",
    syntax: syntax
  })
})