Web Validator by DigestSEO

An MCP server for HTML and CSS validation, technical SEO and accessibility checks, JSON-LD syntax validation, broken-link checks, and responsive screenshots. It is part of the DigestSEO suite of open-source SEO tools.

• Product: digestseo.com/validator-mcp
• Engineering case study: DigestSEO MCP Suite — AI visibility, Search Console, web validation, and trend intelligence
• Documentation: this README
• Support: digestseo.com/support
• Privacy: digestseo.com/privacy

Choose the right surface

This repository contains two deliberately separate MCP surfaces:

Surface	Transport	Best for	Access and side effects
Local npm server	stdio	Claude Desktop, Cursor, and other local MCP clients	Can read user-selected workspace files, contact validation and link targets, and write screenshot files.
Hosted app	Streamable HTTP	ChatGPT and remote MCP clients	Can fetch one authorized public HTML page, run a bounded sitemap-first site audit, or process supplied markup. It cannot access local files, authenticate, recursively crawl links, execute page JavaScript, or create screenshots.

Local installation:

npx -y mcp-web-validator

Hosted endpoint:

https://web-validator-mcp.digestseo.com/mcp

Requirements

• Node.js 22.12.0 or newer
• An MCP client that supports stdio, or a client that supports Streamable HTTP for the hosted endpoint

The validation, SEO, schema, link, and report tools do not download a browser during installation. The local screenshot.capture tool installs its pinned headless browser into Puppeteer's cache on first use, then reuses that browser for later screenshots. Set PUPPETEER_CACHE_DIR if you need a custom cache location.

Tools

The npm package exposes these exact runtime tool names:

Tool	Purpose	Network or filesystem behavior
html.local	Validate a local HTML or XHTML file.	Reads the selected file and submits its markup to the external W3C Nu HTML Checker at validator.w3.org/nu/; .xhtml paths use application/xhtml+xml parsing semantics.
html.url	Validate the markup returned by a public URL.	Fetches the URL, then submits the returned markup to the external Nu checker.
css.local	Validate a local CSS file.	Reads the selected file and submits its CSS to the external W3C Jigsaw CSS Validator.
seo.metadata	Audit titles, descriptions, canonical tags, robots noindex directives, headings, viewport metadata, image alt attributes, and Open Graph metadata.	Processes supplied HTML locally.
links.broken	Check links extracted from supplied HTML.	Sends bounded HTTP requests to eligible public links.
schema.markup	Parse JSON-LD blocks and report JSON syntax errors.	Processes supplied HTML locally.
report.validation	Combine HTML/XHTML, optional CSS, SEO, JSON-LD, and bounded link checks in a Markdown report.	Reads selected files, contacts the validators, and checks eligible public links found in the markup; .xhtml inputs use XHTML parsing semantics for Nu validation.
screenshot.capture	Capture desktop, tablet, mobile, or custom viewport screenshots.	Opens a selected local file or eligible public URL and writes PNG files to the selected output directory; existing matching files may be replaced.

Hosted app tools

The hosted app exposes eight tools:

Tool	Purpose
audit_public_webpage	Fetch one authorized public HTML page and run the combined HTML, SEO/accessibility-signal, and JSON-LD syntax audit. Link checks are optional.
audit_public_site	Run a bounded sitemap-first audit of up to eight authorized, same-origin public pages. It respects robots.txt, returns compact page coverage and deduplicated findings, and does not run site-wide link checks.
validate_html	Validate raw HTML markup already supplied in the conversation.
validate_css	Parse supplied CSS for syntax errors inside the Worker.
audit_seo_metadata	Audit supplied HTML for covered on-page SEO, robots noindex, and accessibility signals.
validate_schema_markup	Check JSON-LD blocks in supplied HTML for JSON syntax errors.
check_broken_links	Check up to 20 authorized public links extracted from supplied HTML.
generate_validation_report	Combine the checks for supplied markup; base_url resolves relative links but does not fetch a page.

audit_public_webpage follows at most three validated redirects, accepts a bounded text/html response, and records the final URL. It does not crawl additional pages, execute JavaScript, authenticate, or fetch linked stylesheets and assets. audit_public_site first locks the final public origin, then reads bounded robots.txt plus same-origin XML Sitemap, RSS 2.0, Atom 1.0, and plain-text sitemap representations before auditing at most eight eligible sitemap pages in one call. Robots discovery may follow up to five validated public redirects across authorities, as defined by the robots protocol; the resulting rules and relative Sitemap directives are still interpreted for the originally audited authority. Sitemap entries and audited pages remain same-origin, and private/reserved destinations, credentials, custom ports, service self-fetches, and HTTPS-to-HTTP redirect downgrades are rejected. The tool never follows HTML links or external sitemap entries; use page_offset when the response says more eligible pages remain. The fetched HTML is sent to https://html5.validator.nu/, the same external Nu HTML Checker used by validate_html and the report tool. CSS parsing, SEO analysis, and JSON-LD parsing run inside the DigestSEO Worker. Optional link checks make capped HEAD requests, with a bounded GET fallback where necessary, to eligible public HTTP(S) URLs; link redirects are reported but not followed. The site tool intentionally does not perform site-wide link checking. The hosted app does not retain tool inputs, fetched HTML, or results.

Privacy Policy

Only validate files, markup, and public webpages that you own or are authorized to inspect. Public page URLs can contain sensitive query values, so remove tokens and private identifiers before submitting them. Do not submit passwords, API keys, access tokens, payment data, health data, private source code, or other sensitive personal data.

The local server runs with the same operating-system permissions as its MCP client. Review tool inputs before approving file access, outbound validation, link checking, or screenshot creation. The hosted app has no access to your local filesystem.

See the published privacy policy for data-handling details and SECURITY.md for private vulnerability reporting.

Configure a local client

Claude Desktop

Add the server to 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"]
    }
  }
}

Restart Claude Desktop after saving the configuration.

Cursor and compatible clients

Create a command/stdio MCP server with:

npx -y mcp-web-validator

This repository also includes a Cursor Marketplace plugin manifest. After the plugin is published, install Web Validator by DigestSEO from Cursor's Marketplace or add the repository from Cursor's plugin UI. The plugin bundles the same local stdio server and does not require API keys or environment variables.

OpenAI Codex

Codex CLI can add the published local stdio package directly:

codex mcp add mcp-web-validator -- npx -y mcp-web-validator

Or add the existing hosted Streamable HTTP surface instead:

codex mcp add mcp-web-validator-hosted --url https://web-validator-mcp.digestseo.com/mcp

Verify either configuration with:

codex mcp list

The local package includes workspace-file validation and responsive screenshot capture. The hosted surface is remote-only and cannot access local files or create screenshots.

Gemini CLI

Gemini CLI supports local stdio MCP servers. Add the published package with:

gemini mcp add mcp-web-validator npx -y mcp-web-validator

This writes the server definition to Gemini CLI's MCP configuration. No API keys or environment variables are required by Web Validator.

GitHub Copilot CLI

Add the published local stdio server with:

copilot mcp add mcp-web-validator -- npx -y mcp-web-validator

When Copilot CLI is started inside a clone of this repository, it can also discover the checked-in root .mcp.json, which already points mcp-web-validator at npx -y mcp-web-validator.

Kiro

This installs the local stdio package, preserving its workspace-file and screenshot capabilities. Node.js 22.12.0+ is required.

Development

Clone the repository and install the locked dependencies:

git clone https://github.com/AKzar1el/mcp-web-validator.git
cd mcp-web-validator
npm ci

Run the local quality gates:

npm run check
npm test
npm run build
npm pack --dry-run

Run the local stdio server:

npm start

Validate the hosted Worker separately:

cd chatgpt-plugin
npm ci
npm run check
npm test
npm run deploy:dry-run

See CONTRIBUTING.md for contribution expectations. Deployment and marketplace-review notes live in chatgpt-plugin/SUBMISSION.md. For agent-assisted installation, see llms-install.md.

Ecosystem

License

Licensed under the MIT License.