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CSS MCP Server

by stolinski
GitHub
JavaScript
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CSS MCP Server

An MCP (Model Context Protocol) server that provides up-to-date CSS documentation from MDN, with intelligent caching.

Features

Simple API - Just pass CSS property names like "grid", "flexbox", or ":has" Official MDN Docs - Fetches documentation directly from MDN's API Markdown Conversion - Converts HTML documentation to clean, readable markdown Smart Caching - SQLite-based cache with 7-day TTL for blazing-fast responses Auto-normalization - Supports both simple slugs ("grid") and full paths ("Web/CSS/grid") Browser Compatibility - Includes browser support data from MDN's BCD

Installation

For MCP Clients (Claude Desktop, etc.)

Add to your MCP settings configuration:

{ "mcpServers": { "css": { "command": "npx", "args": ["-y", "css-mcp"] } } }

For Development

npm install -g css-mcp

Or use with npx:

npx css-mcp --self-test

Requirements

  • Node.js 20+

  • Build tools for native modules (usually pre-installed on most systems)

Usage

Available Tools

get_docs

Fetch CSS documentation for any property, selector, function, or concept.

Parameters:

  • slug (string) - CSS feature name or MDN path

Examples:

// Simple slugs (auto-normalized) get_docs({ slug: "grid" }); get_docs({ slug: ":has" }); get_docs({ slug: "flexbox" }); get_docs({ slug: "@media" }); get_docs({ slug: "::before" }); // Full MDN paths also work get_docs({ slug: "Web/CSS/grid" }); get_docs({ slug: "en-US/docs/Web/CSS/border-radius" });

Returns:

{ "source": "mdn-doc", "slug": "/en-US/docs/Web/CSS/grid", "url": "https://developer.mozilla.org/en-US/docs/Web/CSS/grid/index.json", "title": "grid", "mdn_url": "/en-US/docs/Web/CSS/grid", "summary": "The grid CSS property is a shorthand...", "body": [ { "type": "prose", "title": "Syntax", "content": "The **`grid`** property is a shorthand..." } ] }

get_browser_compatibility

Fetch browser compatibility data for CSS features.

Parameters:

  • bcd_id (string) - Browser Compat Data ID (e.g., "css.properties.grid")

Example:

get_browser_compatibility({ bcd_id: "css.properties.grid" }); get_browser_compatibility({ bcd_id: "css.selectors.has" });

Cache Management

The server automatically:

  • Creates cache at ~/.cache/css-mcp/cache.db

  • Cleans up expired entries on startup

  • Tracks hit counts for each cached entry

  • Uses WAL mode for better concurrent performance

To clear the cache:

rm -rf ~/.cache/css-mcp/

Self-Test

Verify the server is working correctly:

npm test # or css-mcp --self-test # or npx css-mcp --self-test

Expected output:

docs ok (simple slug): { input: 'grid', slug: '/en-US/docs/Web/CSS/grid', ... } docs ok (pseudo-selector + markdown): { input: ':has', ... } bcd ok: { bcd_id: 'css.properties.grid', has_compat: true, ... }

Example: Using with Claude Code

Once configured, you can ask Claude Code:

"Use the CSS MCP to get documentation for flexbox"

"What browser support does :has selector have?"

"Explain how CSS grid works"

Claude will automatically use the MCP to fetch the latest MDN documentation.

Development

# Clone the repository git clone https://github.com/stolinski/css-mcp.git cd css-mcp # Install dependencies npm install # Link for local development npm link # Run tests npm test

Performance

With caching enabled:

  • First fetch: ~400-500ms (network + cache write)

  • Cached fetch: ~100ms (~5x faster)

  • Cache size: ~390KB for typical usage

Troubleshooting

"Module did not self-register"

This usually means native modules need rebuilding:

npm rebuild better-sqlite3

Cache not working

Check cache directory permissions:

ls -la ~/.cache/css-mcp/

Should show cache.db, cache.db-shm, and cache.db-wal files.

Contributing

This is an experimental MCP server for CSS documentation tooling, let's work on tools that make AI better at CSS

This server cannot be installed

-
security - not tested
-
license - not tested
-
quality - not tested

How are these scores calculated?

hybrid server

The server is able to function both locally and remotely, depending on the configuration or use case.

Provides up-to-date CSS documentation and browser compatibility data from MDN through natural language queries. Features intelligent caching and supports all CSS properties, selectors, functions, and concepts with automatic normalization.

  1. Features
    1. Installation
      1. For MCP Clients (Claude Desktop, etc.)
      2. For Development
    2. Requirements
      1. Usage
        1. Available Tools
      2. Cache Management
        1. Self-Test
          1. Example: Using with Claude Code
            1. Development
              1. Performance
                1. Troubleshooting
                  1. "Module did not self-register"
                  2. Cache not working
                2. Contributing

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