The Nu Html Checker (vnu) is a production-grade, multi-format web standards validator that checks HTML, CSS, SVG, and other document types for conformance to W3C specifications. It operates as a Java library, command-line tool, HTTP server (servlet), or Docker container. The codebase integrates several independently developed modules: an HTML5 parser (based on Mozilla's parser), a CSS validation engine (W3C CSS Validator), a RELAX NG schema validator (jing-trang), a URL canonicalization library (galimatias), and a language detection component. Validation is driven by a combination of RELAX NG schemas, DTDs, XSD datatypes, and hardcoded CSS property rules. The system is actively maintained and widely used, with a strong focus on correctness and standards compliance. Its architecture is modular but tightly coupled internally, reflecting its evolution from multiple W3C projects.