For example I am willing to develop in your browser if it meant I wouldn't broke some rules and the browser would care about that. Of course html validators exist and browsers display invalid css in their inspectors but I am asking for something like that would expect valid code. throw an error in my face if I didn't close a html tag or used an invalid css value, at any point.

Like if a javascript code resulted in my page getting invalid layout then browser could yell at me that causes an invalid layout. Maybe even a small performance improvement if my code conformed to every layout rule.