CSS for AI-assisted teams

Build quickly.
Keep every class predictable.

Nucleus is a polished, accessible CSS framework with 697 documented classes, zero runtime JavaScript, and machine-readable metadata so coding assistants use it accurately.

Install Nucleus Explore the system

697: classes
76: tokens
60: components
0 KB: runtime JS

<article class="n-card n-card-interactive">
  <div class="n-card-body n-stack">
    <p class="n-overline">New</p>
    <h2>Readable by default</h2>
    <button class="n-btn n-btn-primary">
      Continue
    </button>
  </div>
</article>

New

Readable by default

Meaningful classes. Finished states. No runtime.

Why Nucleus

A small framework with unusually complete documentation.

No invented classes

The manifest describes every valid class, component, state, and token. The validator catches anything that does not exist.

Drop in one stylesheet

Use npm, a CDN, or modular imports. There is no configuration file, runtime package, or required JavaScript.

Accessibility is part of the default

Focus states, reduced motion, high contrast, dark mode, native controls, and print behavior ship with the framework.

Live playground

Change the system, not the markup.

Theme and density tokens cascade through real Nucleus components.

Theme Density

Live component

Ship a clear interface.

Tokens update the whole composition without changing a class name.

Documentation

Go from layout to finished interface.

LayoutGrid and intrinsic patternsResponsive columns, containers, stack, cluster, split. ComponentsFinished UI building blocksAlerts, tables, navigation, avatars, feedback. FormsAccessible inputs and statesLabels, validation, controls, and complete forms. UtilitiesPredictable composition helpersSpacing, type, display, color, effects, and state.

Install

Start with one command or one link.

Version 2.0.0. Prefix every class with n- and it will not collide with your application CSS.

npm

npm install @navitallabs/nucleus-css

Then import @navitallabs/nucleus-css once in your application entry point.

CDN

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@navitallabs/nucleus-css@2.0.0/dist/nucleus.min.css">

Pin the version in production so future releases cannot change your UI unexpectedly.

Don't want to type commands yourself? Paste this into Claude, Cursor, Codex, ChatGPT, or any AI coding assistant and let it install and wire up Nucleus for you.

Prompt for your AI

Install Nucleus CSS (the @navitallabs/nucleus-css package) in this project. Run `npm install @navitallabs/nucleus-css`, then import it once in the app's entry point (or link dist/nucleus.min.css if this is a plain HTML page). If you support project rules, run `npx @navitallabs/nucleus-css ai-setup <your-tool>` (claude, codex, cursor, or antigravity) — otherwise read node_modules/@navitallabs/nucleus-css/llms.txt before writing any markup. Build every element using only documented `n-`-prefixed Nucleus classes; never invent a class name or reuse another framework's naming convention. Before you finish, run `npx @navitallabs/nucleus-css validate ./src` and fix anything it reports.

AI setup

Give each coding assistant the right Nucleus rules.

One command writes the shared class context and the native project rule for your tool.

Claude Code

Creates a scoped rule in .claude/rules.

npx @navitallabs/nucleus-css \
  ai-setup claude

Codex

Adds an idempotent Nucleus block to AGENTS.md.

npx @navitallabs/nucleus-css \
  ai-setup codex

Cursor

Creates an always-applied rule in .cursor/rules.

npx @navitallabs/nucleus-css \
  ai-setup cursor

Antigravity

Creates a workspace rule in .agents/rules.

npx @navitallabs/nucleus-css \
  ai-setup antigravity

Machine-readable by design

Give coding tools the same source of truth.

Nucleus publishes concise and full LLM references, a JSON manifest, and a CLI validator alongside the human documentation.

llms.txt Concise assistant context llms-full.txt Complete class reference JSON manifest Structured framework metadata Manifest schema Roles, aliases, and structured queries Class reference All 697 documented classes