This program validates the html of an entire website at the press of a button. It's free. No ads, no tricks. I made it because I needed it at work and thought I might as well share it with others.
You could use W3C Markup Validation Service url: https://validator.w3.org/
You should install vnu, running it on your computer in order to avoid getting black listed at the W3C Markup Validation Service due to heavy traffic:
java -cp vnu.jar nu.validator.servlet.Main 8888
After validation a report is displayed and saved in c:\users\<username>\Website Validator report. Make sure to copy reports you want to save, otherwise it will be replaced the next time you use WV.