HTML_CodeSniffer

HTML_CodeSniffer is a client-side script that checks HTML source code and detects violations of a defined coding standard. It is written entirely in JavaScript, needs no server-side processing and can be extended with custom “sniffs” to enforce custom coding standards.

It comes with standards for the three conformance levels of the Web Content Accessibility Guidelines (WCAG) 2.1 and for the web-related parts of the U.S. Section 508 legislation. An auditor interface, launched from a bookmarklet, lets users run these accessibility checks on any page. HTML_CodeSniffer is also used as a test runner by other tools, such as Pa11y.

HTML_CodeSniffer is developed by Squiz and is free and open source under the BSD-3-Clause licence. Its code has not changed since 2024.