There are many testing sites to choose from. We recommend: 1. W3C Markup Validation Service checks documents written in HTML, XHTML, SMIL, MathML and more. 2. Jest allows you to take test snapshots to keep track of large projects. 3. Selenium automates web apps and other administration tasks. 4. Headless Chrome has no graphics display, so all features are rendered to the command line. This is ideal for automated testing.