@nagi-labs/nagi-css-core

Nagi CSS

CSS, after the wind. Nagi CSS is a lint-enforced structural contract for component-owned CSS. It derives names from explicit evidence, validates applicable definitions, and makes Predefined, Defined, Unregistered, and Structural names visible. Lint and measurement use the same analysis.

Website · Documentation · Contract

It keeps semantic classes and selectors in component-owned CSS, with no runtime of its own. Plain CSS is the stable declaration backend. An experimental, explicit tailwind-apply compatibility backend lets an owned implementation use Tailwind @apply without putting utility classes in its template. One ESLint plugin checks the component template and its <style> blocks together across Vue, Nuxt, Svelte, and Astro.

Install

Role names determine base classes; purposes add static variants without replacing the base. For example, data-role="range/fill" uses .fill, while data-purpose="pagination/next" uses .button.-next or .link.-next. Projects and libraries can register either kind through the same scoped definition format. Purpose definitions may constrain their target HTML element or ARIA role. Existing HTML class names are unchanged.

Requirements:

• Node.js 22.18 or newer
• ESLint 9 or newer
• component-owned <style> blocks whose selectors remain statically readable

npm install --save-dev @nagi-labs/eslint-plugin-nagi-css

Using pnpm or Vite+? Use the equivalent development-dependency command for your package manager.

Append Nagi CSS after the framework's official flat config:

import nagiCss from "@nagi-labs/eslint-plugin-nagi-css"

export default [
  // The framework's official ESLint config comes first.
  ...nagiCss.configs.recommended({
    surfaceRootPrefixes: ["app-"],
  }),
]

Run the application's normal ESLint command. If it does not have one yet:

vp exec eslint .

eslint --fix applies only changes whose answer the contract can derive unambiguously.

Framework-specific setup:

• Vue
• Nuxt
• Svelte
• Astro

The setup index covers shared installation, design tokens, incremental adoption, and verification.

Try the example

Open the Vue example in StackBlitz, or clone this repository and change into examples/vue-minimal. The example pins the published npm plugin and can be installed, linted, built, and run without resolving this repository's workspace packages. Detailed local commands and intentional breakage exercises are in the example README.

What it enforces

Surface ownership, base role, variants/state and CSS structure form one contract:

• the surface root from the configured prefix and component filename;
• fixed classes from HTML elements and configured UI components;
• internal roles classified as Predefined, Defined, Unregistered, or Structural;
• native/ARIA Predefined names and described Built-in Anatomy/Custom definitions;
• scoped JSON role definitions with optional role descriptions, selected by static data-role="scope/role" markers, with required declaration checks;
• unregistered semantic names allowed with warnings while CSS validation continues;
• static modifiers, attribute state, and legitimate shared STN sibling selectors;
• selectors that mirror owned DOM with > and stop at component boundaries;
• semantic token references for colors and repeated design-system scale values, while component geometry stays plain CSS and genuine one-off optical corrections use descriptive --local-* values.

<template>
  <section class="app-user-card">
    <span class="icon" :class="iconName" />
    <div class="value" :data-active="status === 'active'">Ada Lovelace</div>
  </section>
</template>

<style scoped>
.app-user-card {
  > .icon {}
  > .value[data-active="true"] {
    color: var(--color-text);
  }
}
</style>

Nagi CSS extends the framework's official ESLint config instead of replacing its parser, globals, or generated settings. A separate configuration file is not required for normal application setup.

Visual visibility and accessibility-tree exposure remain separate. ARIA states may be selected when they already describe real component state, but ARIA must not be invented as a styling hook. Content that is visually concealed while remaining available to assistive technology is styled through its derived base selector rather than an -assistive or -sr-only class. See Visual hiding and the accessibility tree.

Define and measure

The redesigned source API accepts roleDefinitions containing scoped JSON objects with optional layer and declaration fields on roles and purposes. Purposes may use on: { element: "button" } or on: { aria: "button" } to restrict their target; on does not assign HTML or ARIA semantics. The previous role key remains a compatibility alias; do not specify both keys. They default independently to "implementation" and "optional". Use declaration: "required" for static declaration checks, not runtime visibility or behavioral guarantees. The legacy boolean required remains a compatibility input; contradictory settings are rejected.

roleDefinitions accepts objects or independent local JSON paths. loadRoleDefinition reads JSON safely; data-role="scope/root" establishes an instance and custom roles use the same fully qualified attribute without adding ARIA behavior. See the definition reference and Vue/Svelte/Astro examples.

nagi-css measure --config nagi.config.mjs --cwd . --json reports internal styled declarations as Predefined / Defined / Unregistered / Structural, keeping surfaces, boundaries and unknowns separate. Predefined names and author-selected definitions are reported separately. The 0.7.0 migration guide covers the new APIs and configuration diagnostics; the pinned Vue starter still uses 0.5.1 until publication. Maintainers verify candidate tarballs in isolated consumers before publishing, as described in the release guide. Unresolved behavior bundles remain explicit diagnostics; adopting these APIs does not require a UI library to redesign its behavior API. Consumer migration findings are assessed separately from analyzer defects and package failures.

Documentation

• Definition model migration

• Definition and measurement specification

• Built-in Anatomy definitions

• Contract — the complete naming, selector, ownership, and value rules

• FAQ — design rationale, tradeoffs, and comparisons

• Configuration reference — UI libraries, slots, severity, emit policy, and tokens

• Agent instructions — portable rules for agents editing components

• Agent skill — the complete generate-and-verify workflow

• Migrating to 0.5 — component-name matching and stacking diagnostics

• Migrating to 0.4 — peer-based variant rules

• Migrating to 0.3 — lint behavior and configuration changes

Packages

• @nagi-labs/eslint-plugin-nagi-css — standard template and component-style integration
• @nagi-labs/nagi-css-core — shared configuration and semantic analysis
• @nagi-labs/nagi-css — optional standalone runner for external configurations

The ESLint plugin is the normal entry point. The standalone runner supports external configurations and projects that use another primary linter.

Scope

Nagi CSS checks Vue, Svelte, and Astro component templates together with their CSS <style> blocks. Nuxt is supported through Vue's parser and Nuxt's generated ESLint config. The default plain declaration mode needs no additional CSS compiler. tailwind-apply requires the application to provide Tailwind's build integration and remains experimental; its coverage and API may change before it is promoted to a stable backend.

Preprocessor syntax and standalone .css files are outside the component-owned contract. Global resets, token declarations, and cross-surface exceptions remain the application's responsibility. See the contract for the exact boundary.

Acknowledgements

Nagi CSS would not exist in its current form without RSCSS by Rico Sta. Cruz.

RSCSS showed me that a small set of conventions—thinking in components, naming elements locally, and using direct-child selectors to protect component boundaries—could make CSS dramatically easier to reason about.

Nagi CSS takes those ideas in a more mechanically enforceable direction, deriving names from HTML and checking ownership boundaries statically. But its starting point is unmistakably RSCSS.

Thank you, Rico, for publishing an approach that has shaped how I think about CSS for years.

Development

Install dependencies and run the test suite through Vite Plus:

vp install
vp run test

See CONTRIBUTING.md before changing vocabulary or rule behavior. Maintainers should follow the release guide when publishing packages.

License

MIT, by nagi-labs contributors.