mcp-web-validator

Part of the DigestSEO suite of open-source SEO tools.

Product home: digestseo.com/validator-mcp · Support: digestseo.com/support

The public, read-only ChatGPT app is hosted at https://web-validator-mcp.digestseo.com/mcp. It accepts HTML, CSS, and authorized public URLs you explicitly supply; the local MCP described below remains the option for workspace files and responsive screenshots.

A Model Context Protocol (MCP) server that empowers AI coding assistants (like Claude, Cursor, ChatGPT, etc.) to validate HTML/CSS markup against the official W3C specification engine and perform technical SEO/accessibility audits directly inside local workspaces.

💬 How It Works (Visual Flow)

Imagine asking your AI coding assistant:

User: "I just finished building my homepage. Can you validate it, check for SEO issues, and take responsive screenshots to see if it looks correct?"

Here is the exact flow of how the assistant uses mcp-web-validator to audit the code and present results:

1️⃣ Step 1: Technical & SEO Score Card

The assistant automatically runs generate_validation_report which aggregates standard specs and outputs a unified PageSpeed-style scorecard directly in your chat:

📋 Web Validation & SEO Audit Report — 🟢 92/100

Generated for: index.html

⚡ Page Health Scores (PageSpeed Inspired)

| Score Card | Status | Score | | :--- | :---: | :---: | | W3C HTML Validation | 🟠 Needs Work | 88 / 100 | | SEO & Accessibility | 🟠 Warnings | 88 / 100 | | Links Integrity | 🟢 All Good | 100 / 100 |

2️⃣ Step 2: Responsive Viewport Screenshot Audits

The assistant launches local Puppeteer using capture_screenshots to render the page across multiple devices:

| Desktop (1440x900) | Tablet (768x1024) | Mobile (375x812) | | :---: | :---: | :---: | | 🖥️ desktop.png | 📟 tablet.png | 📱 mobile.png |

✨ Features

• HTML Validation: Leverages the official W3C Nu HTML Checker API (no API keys required).
• CSS Validation: Calls the W3C Jigsaw CSS Validator API to debug stylesheets.
• Technical SEO Check: Audits title/description lengths, viewport responsiveness, heading hierarchies, Open Graph metadata, and image alt attributes.
• Broken Link Checker: Scans HTML documents for broken internal or external URLs.
• Schema Validator: Parses and validates embedded JSON-LD structured schemas.
• Visual Viewport Audits: Uses a headless Puppeteer browser locally to capture responsive layout screenshots (desktop, tablet, mobile) for design auditing.

🛠️ Tools Exposed

1. validate_local_html
   • Arguments: filePath: string (Absolute or relative path to the local HTML file)
   • Description: Submits local HTML to the W3C Nu Checker and returns syntax errors/warnings.
2. validate_url
   • Arguments: url: string (The live public URL to audit)
   • Description: Validates live page markup against W3C standards.
3. validate_local_css
   • Arguments: filePath: string (Absolute or relative path to the CSS file)
   • Description: Checks local CSS syntax against the W3C Jigsaw API.
4. audit_seo_metadata
   • Arguments: htmlContent: string (Raw HTML string)
   • Description: Performs a fast, local SEO audit (titles, descriptions, headings, images, alt tags).
5. check_broken_links
   • Arguments: htmlContent: string, baseUrl?: string
   • Description: Tests all links in the document to detect 404s or broken URLs.
6. validate_schema_markup
   • Arguments: htmlContent: string
   • Description: Validates JSON-LD schema syntax.
7. generate_validation_report
   • Arguments: htmlFilePath: string, cssFilePath?: string, baseUrl?: string
   • Description: Automatically runs all checks (HTML/CSS syntax, SEO audit, JSON-LD, broken links) and generates a unified Markdown report card styled with PageSpeed-inspired scoring.
8. capture_screenshots
   • Arguments: targetPath: string (Path to local HTML file or HTTP(S) URL), outputDir?: string, viewports?: Array<{ name: string, width: number, height: number }>
   • Description: Renders the target in a local headless browser and saves screenshots across desktop, tablet, and mobile views.

🚀 Configuration

1. Claude Desktop

Add this to your claude_desktop_config.json:

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

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

2. Cursor

To use this inside Cursor, navigate to Settings > Features > MCP:

1. Click + Add New MCP Server.
2. Set Name: mcp-web-validator
3. Set Type: command
4. Set Command: npx -y mcp-web-validator

3. Development / Local Execution

To run and test the server locally:

git clone https://github.com/AKzar1el/mcp-web-validator.git
cd mcp-web-validator
npm install
npm run build
node dist/index.js

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.