W3C MCP Server

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

Related MCP server: @shuji-bonji/web-compat-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)

get_spec_dependencies

Get basic information for a specification.

Note: Dependency data (dependencies / dependents) is not yet exposed by the upstream web-specs package, so these fields currently return empty arrays. Only the base spec metadata is reliable for now.

Parameters:

• shortname (required): Specification 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:

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