W3C offers tools and validators including:

Nu HTML Checker: Checks HTML documents.
Link Checker: Checks web pages for broken links.
Internationalization Checker: Checks level of internationalization
EPUBCheck: Conformance checker for EPUB publications.
CSS Validator: Checks your CSS.
RDF Validator: Checks and visualizes RDF documents.
RSS Feed Validator: Validator for syndicated feeds (RSS and Atom feeds).
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https://www.w3.org/developers/tools/