The w3c-mcp server provides AI assistants with programmatic access to official W3C, WHATWG, and IETF web standards data through the Model Context Protocol (MCP).

Core Capabilities:

• Specification Discovery: List, search, and retrieve detailed information about web specifications with filtering by organization, keyword, category, or query string. View specification status, URLs, repository, test info, and dependency relationships (both dependencies and dependents)

• WebIDL Access: Get JavaScript API interface definitions (WebIDL) for specific specifications or list all specs with WebIDL available

• CSS Properties: Query CSS property definitions from specific or all CSS specifications, search properties by name, and list all CSS specs with property definitions

• HTML Elements: Retrieve HTML element definitions from specific or all HTML/SVG specifications, and search for specific elements by name

• PWA Standards: Access Progressive Web App related specifications including core PWA specs (Service Worker, Web App Manifest, Push API, Background Sync, Notifications, etc.)

Technical Features:

• Uses official W3C-maintained data packages (web-specs, @webref/idl, @webref/css, @webref/elements) for accurate, machine-readable standards data

• Fast startup (~70ms) with optimized search and O(1) spec lookups

• Works with Claude Desktop and Cursor via MCP protocol configuration

• Includes debug and performance logging options for development

Provides tools to list and retrieve CSS property definitions, values, and specifications from official W3C web standards.

Enables discovery and detailed information retrieval for Progressive Web App (PWA) related specifications like Service Worker and Web App Manifest.

Provides access to SVG element definitions and specification metadata from official web standards.

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

npx @shuji-bonji/w3c-mcp

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

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

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

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

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

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

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

# 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

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

# 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