HTML_CodeSniffer is a JavaScript library and audit tool that scans HTML pages (in‑browser, via bookmarklet, command‑line or Node.js) for violations of accessibility and presentation standards such as WCAG 2.x and U.S. Section 508, and can be extended with custom rule sets. It’s aimed at web developers, QA engineers, and accessibility testers who need automated checks and actionable feedback to ensure their sites meet accessibility guidelines.