Pa11y is a mature, widely-used automated accessibility testing tool that runs via command-line interface or Node.js API. It drives a headless Chrome browser (via Puppeteer) to load web pages, execute optional user interactions (actions), and then run one or more accessibility test runners (HTML_CodeSniffer and axe-core) against the page. Results are reported in multiple formats (CLI, CSV, JSON, HTML, TSV) and can be filtered by severity, ignored rules, or threshold. The architecture is a monolith library with a thin CLI wrapper, but the runner and reporter systems are modular and extensible via npm packages or local files. Key entry points are `bin/pa11y.js` for CLI and `lib/pa11y.js` for programmatic use. The data flow: launch Chrome, navigate to URL, run actions, inject runner scripts into the page, evaluate accessibility, collect and process issues, output via selected reporter. External dependencies include Puppeteer, axe-core, @pa11y/html_codesniffer, commander, and several utility libraries. The codebase is well-structured, thoroughly tested (unit and integration), and follows modern Node.js practices with async/await and ES6+ features.