HTML Validator : Can we report issues based on HTML Validator tool?
Seriously, whether you can use an HTML validator tool as an oracle depends on whether the application under test uses custom attributes or not; whether the AUT is generating code from a framework such as Angular; whether the AUT puts the scripting inline or has it separated; and many other factors around how the AUT goes from code to displayed HTML.
I’ve yet to see an HTML validator that can work with even simple websites without producing an unacceptable number of false positives.
If your AUT is simple enough and uses standard enough HTML, you may be able to use the validator tool to identify and report issues. I think it’s more likely that if you use one, it will be to help you find issues caused by other factors.
I would ask yourself the question “What pressing business problem will I be solving by validating HTML? Am I the only one on the team that thinks this is a good idea or are there any other people seeing the same problem”. If you have a good reason for it, do it, othwerise move on to the next most important thing on the list.