W3c

npm version License: MIT Node.js MCP Built with Claude Code

日本語版 README

MCP Server for accessing W3C/WHATWG/IETF web specifications. Provides AI assistants with access to official web standards data including specifications, WebIDL definitions, CSS properties, and HTML elements.

Installation

npm install -g @shuji-bonji/w3c-mcp

Or use directly with npx:

npx @shuji-bonji/w3c-mcp

Configuration

Claude Desktop

Add to your Claude Desktop configuration file:

macOS: ~/Library/Application Support/Claude/claude_desktop_config.json Windows: %APPDATA%\Claude\claude_desktop_config.json Linux: ~/.config/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "w3c": {
      "command": "npx",
      "args": ["-y", "@shuji-bonji/w3c-mcp"]
    }
  }
}

Claude Code

Add the server using the claude mcp CLI:

claude mcp add w3c -- npx -y @shuji-bonji/w3c-mcp

Or edit ~/.claude.json / project-level .mcp.json manually with the same mcpServers block shown above.

Cursor

Add to your Cursor MCP settings (.cursor/mcp.json in your project or global settings):

{
  "mcpServers": {
    "w3c": {
      "command": "npx",
      "args": ["-y", "@shuji-bonji/w3c-mcp"]
    }
  }
}

Available Tools

Specification Discovery

list_w3c_specs

List W3C/WHATWG/IETF web specifications with optional filtering.

Parameters:

• organization (optional): Filter by organization - "W3C", "WHATWG", "IETF", or "all"
• keyword (optional): Filter by keyword in title or shortname
• category (optional): Filter by category
• limit (optional): Maximum number of results (default: 50)

get_w3c_spec

Get detailed information about a specific web specification.

Parameters:

• shortname (required): Specification shortname (e.g., "service-workers", "appmanifest", "fetch")

search_w3c_specs

Search web specifications by query string.

Parameters:

• query (required): Search query (e.g., "service worker", "manifest", "storage")
• limit (optional): Maximum number of results (default: 20)

WebIDL

get_webidl

Get WebIDL interface definitions for a specification. WebIDL defines the JavaScript APIs.

Parameters:

• shortname (required): Specification shortname (e.g., "service-workers", "fetch", "dom")

list_webidl_specs

List all specifications that have WebIDL definitions available.

CSS

get_css_properties

Get CSS property definitions from a specific spec or all specs.

Parameters:

• spec (optional): Specification shortname (e.g., "css-grid-1", "css-flexbox-1")
• property (optional): Search for a specific CSS property by name

list_css_specs

List all CSS specifications that have property definitions available.

HTML Elements

get_html_elements

Get HTML element definitions from a specific spec or all specs.

Parameters:

• spec (optional): Specification shortname (e.g., "html", "svg")
• element (optional): Search for a specific element by name (e.g., "video", "canvas")

list_element_specs

List all specifications that have HTML element definitions available.

PWA

get_pwa_specs

Get all Progressive Web App (PWA) related specifications.

Parameters:

• coreOnly (optional): If true, return only the core PWA specs (Service Worker, Manifest, Push, Notifications)
• includeRelated (optional): If true, also include related specs whose title mentions storage, caching, offline, etc. (Storage, Storage Buckets, Storage Access API, ...). Ignored when coreOnly is true.

By default only the specs listed in PWA_SHORTNAMES (src/constants/index.ts) are returned, matched by exact shortname.

Usage Examples

Find Service Worker APIs

Use the get_webidl tool with shortname "service-workers" to see the ServiceWorker interface definitions.

Explore PWA Technologies

Use get_pwa_specs to see all PWA-related specifications, then use get_w3c_spec for details on each one.

Look up CSS Grid Properties

Use get_css_properties with spec "css-grid-1" to see all CSS Grid layout properties.

Search for Storage APIs

Use search_w3c_specs with query "storage" to find all storage-related specifications.

Data Sources

This MCP server uses the following W3C/webref data packages:

Package	Description
web-specs	Metadata for all web specifications
@webref/idl	WebIDL interface definitions
@webref/css	CSS properties and values
@webref/elements	HTML element definitions

These packages are maintained by the W3C and provide machine-readable data extracted from official specifications.

GitHub Repositories:

• https://github.com/w3c/browser-specs
• https://github.com/w3c/webref

Debug Mode

Enable debug logging with environment variables:

# Enable debug logging
W3C_MCP_DEBUG=true npx @shuji-bonji/w3c-mcp

# Enable performance logging only
W3C_MCP_PERF=true npx @shuji-bonji/w3c-mcp

Debug output includes:

• Tool call arguments
• Execution timing
• Data loading performance

Architecture

src/
├── index.ts           # MCP server entry point
├── constants/
│   └── index.ts       # Centralized configuration constants
├── data/
│   └── loader.ts      # Data loading with caching
├── tools/             # Tool implementations
│   ├── list-specs.ts
│   ├── get-spec.ts
│   ├── search-specs.ts
│   ├── get-webidl.ts
│   ├── get-css.ts
│   ├── get-elements.ts
│   └── get-pwa-specs.ts
├── schemas/
│   └── index.ts       # Zod validation schemas
├── errors/
│   └── index.ts       # Custom error classes
├── utils/
│   ├── logger.ts      # Debug logging utilities
│   ├── mapper.ts      # Spec data mapping utilities
│   ├── search.ts      # Generic search utilities
│   └── suggestions.ts # Suggestion generation utilities
└── types/
    └── index.ts       # TypeScript type definitions

tests/
├── setup.ts           # Test setup
├── data/              # Data loader tests
├── tools/             # Tool tests
└── integration/       # MCP server integration tests

Performance

• Startup: ~70ms parallel preloading of all data
• Spec Lookup: O(1) using Map-based index
• Search: Optimized with early termination on exact matches

Development

# Clone the repository
git clone https://github.com/shuji-bonji/w3c-mcp.git
cd w3c-mcp

# Install dependencies
npm install

# Build
npm run build

# Run in development mode
npm run dev

# Run with debug logging
W3C_MCP_DEBUG=true npm start

# Run tests
npm test

# Run tests with coverage
npm run test:coverage

# Lint code
npm run lint

# Format code
npm run format

# Lint + format (auto-fix)
npm run check

License

MIT