W3C Validator is a free HTML code validation tool by W3C. W3C provides multiple options to validate the HTML code in the website. You have to choose based on your need. This tool will help to validate the HTML code against W3C’s HTML standards.
The W3C online validator accepts URL only from public IP. Also we can validate one page at a time (http://validator.w3.org/). So this tool is commonly used for public websites with few pages.
W3C is providing an experimental web service API. But this has limited usage, and W3C is not recommending this option for extensive usage (http://validator.w3.org/docs/api.html).
For internal validation purpose we can download the W3C validator source code and install it at our environment and do the validation. W3C recommends this option for extensive validation and usage. This will be the best option for Intranet and websites with lot of pages. It will be also helpful to validate the HTML code in the .net web applications. For installing Local instance of W3C validator you need Apache and Perl environment as a base prerequisite. The installation details are available in http://validator.w3.org/docs/install_win.html.
See this article for the other code review tools I’ve used for .Net web application development.