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w3c-mcp

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TypeScript
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W3C MCP Server

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

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

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

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 dependency information for a specification.

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:

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:

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 ├── 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 └── 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

This server cannot be installed

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security - not tested
A
license - permissive license
-
quality - not tested

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