Acceptable Validation Errors Ideally all your pages would be 100% valid, though it’s not practical or even desirable at times to achieve 100% validation.
Some code that’s commonly used just won’t validate.
Vender specific prefixes such as -webkit and -moz will never validate by definition. That doesn’t mean you shouldn’t use those vendor specific prefixes.
Hacks for specific browsers (I’m looking at you Internet Explorer) also won’t validate, though it’s unlikely you’ll find a developer who’s never written a coding hack. Sometimes they’re all you have to get the page working right.
Validators will also issues warnings in addition to errors. 100% valid code means 0 errors, but it could include any number of warnings. Some of those warnings aren’t going to be things you’ll want to fix. Many tracking parameters you add to the end of a url will give a warning for example.
In the end you shouldn’t sweat it if your pages aren’t 100% valid. Some errors and many warnings are perfectly acceptable as part of a well-coded page.