CSE HTML Validator Lite is a fast, easy to use, and FREE (for personal and educational use) HTML development tool for Microsoft Windows that helps you create problem-free HTML and XHTML documents. It's also a great learning tool.
CSE HTML Validator Professional is a fast, powerful, user configurable, and easy to use professional web development tool that helps you create syntactically correct and accessible HTML, XHTML, and CSS. Can automatically fix HTML. Also checks links.
