SortSite

SortSite is a desktop application that scans entire websites for accessibility, HTML/CSS validity, and link integrity issues. It supports WCAG 2.1 and 2.2 guidelines and provides detailed reports with remediation advice. While not strictly open-source, it offers a free trial and a relatively affordable license. Its particularly useful for large websites or organizations that need to audit multiple sites regularly.

It's a good option for those who prefer a desktop application over a browser extension or command-line tool.