The system is the W3C Nu HTML Checker (vnu), a production-grade conformance checker for HTML, CSS, SVG, and related web standards. It operates as a command-line tool (vnu.jar), a web service (HTTP API on port 8888), a Docker image, and an embeddable Java library. The architecture is a monolith composed of several integrated sub-projects: an HTML parser (validator.nu), a CSS validator (org.w3c.css), a RELAX NG schema validator (jing-trang), a URL canonicalization library (galimatias), and a language detection module (langdetect). Key entry points include the servlet (nu.validator.servlet.Main), the CLI wrapper (vnu-jar.js), and the Python build orchestrator (checker.py). Data flows involve parsing input documents, applying validation rules from W3C specifications, and emitting structured error/warning messages in various formats (JSON, XML, text). External dependencies include Java (JDK 17+), Ant, Maven, and network access for schema downloads. The codebase demonstrates several well-crafted patterns: a decorator-based validator wrapper system for filtering/modifying validation behavior (e.g., AriaAttributeDroppingValidatorWrapper), a clean URLCanonicalizer interface with multiple implementations, utility classes like ForkContentHandler for multiplexing SAX events, and a consistent property-class pattern for CSS validation. However, it also contains significant boilerplate duplication across CSS property classes and some stub implementations that throw exceptions, indicating areas of incomplete coverage. The repository is actively maintained by the W3C with regular commits and CI, though some vendored components and Python 2 example scripts show signs of technical drift.