I think it's ES5's strict mode possibly? I'm not entirely sure. I prefer to code things as strictly as possible to ensure my code will work no matter what. I always keep the strict warnings enabled, those errors are generally not difficult to fix. I recommend keeping your error reporting as sensitive as it'll get (both client and server side) as it'll encourage you to write better more secure code.
If you code everything and serve is as XHTML served as application/xhtml+xml the only thing you'll need a validator for it to check for duplicate id's and not that if you have good coding practices. ;)