Web Validator MCP server

What it does

This MCP server validates HTML and CSS against W3C standards, audits on-page SEO signals and accessibility attributes, checks JSON-LD syntax, verifies links, and captures responsive screenshots. It is designed for developers and SEO professionals who need to validate web pages and audit technical aspects of websites.

• Validate HTML markup and CSS files against W3C standards to identify syntax errors and compliance issues.
• Audit on-page SEO metadata including titles, descriptions, headings, alt attributes, and Open Graph tags.
• Check for broken links in a page and capture responsive screenshots at different viewport sizes.

Tools this server exposes

Tool	Does
html.local	Validate a local HTML file against the W3C Nu HTML Checker.
html.url	Validate HTML markup returned by a public URL against the W3C Nu HTML Checker.
css.local	Validate a local CSS file against the W3C Jigsaw CSS Validator.
seo.metadata	Audit on-page SEO signals including titles, descriptions, canonical tags, headings, viewport metadata, image alt attributes, and Open Graph metadata.
links.broken	Check links extracted from supplied HTML for availability and response status.
schema.markup	Parse JSON-LD blocks and report JSON syntax errors.
report.validation	Combine HTML, CSS, SEO, JSON-LD, and link checks into a Markdown report.
screenshot.capture	Capture desktop, tablet, mobile, or custom viewport screenshots of local files or public URLs.
audit_public_webpage	Fetch a public HTML page and run combined HTML, SEO, and JSON-LD validation with optional link checks.
audit_public_site	Run a bounded sitemap-first audit of up to eight public pages from the same origin.
validate_html	Validate raw HTML markup supplied in the conversation.
validate_css	Parse supplied CSS for syntax errors.
audit_seo_metadata	Audit supplied HTML for on-page SEO and accessibility signals.
validate_schema_markup	Check JSON-LD blocks in supplied HTML for JSON syntax errors.
check_broken_links	Check up to 20 public links extracted from supplied HTML.
generate_validation_report	Combine HTML, CSS, SEO, and JSON-LD checks for supplied markup.

Before you enable it

An MCP server runs with whatever access you give it. Start with a development account rather than production, and check which tools the server exposes before you let an agent call them unattended.