HTML_CodeSniffer is a coding standards checker for websites. Users can run HTML_CodeSniffer inside their browser by adding a bookmarklet to their bookmarks bar. The sample public version will check a site for conformance with the Web Content Accessibility Guidelines (WCAG) version 2.0. Users simply visit a site and click their CodeSniffer bookmark to generate an interactive report of any issues found. As users select individual issues within the report, CodeSniffer indicates the relevant visual element on the page, displays the problematic markup and offers a link to suggested techniques for resolving the issue. Developers can clone the BSD-licensed HTML_CodeSniffer source from GitHub and create their own customized versions that include any additional checks they want. The HTML_CodeSniffer bookmarklet works in Google Chrome, Firefox, Safari, and Internet Explorer. Additional ways of running HTML_CodeSniffer, including command-line use with a headless browser, are described on the GitHub page.