Click the More Options link to see on each page to see.There are plenty of browser extensions that will test the page you’re viewing in the W3C validators.Fix those one or two errors and watch as many more errors are gone when you revalidate the page.Ideally all your pages would be 100% valid, though it’s not practical or even desirable at times to achieve 100% validation.Some will probably tell you that valid code will help your pages rank better. Other than show stopping errors (which validation can help you find) search engines really don’t care much about your code.
For example forgetting to close a tag early in your document could end up resulting in dozens, even hundreds of errors.
It should work across more browsers and devices making it more usable and accessible.
Different browsers may treat invalid code differently and sometimes in unexpected ways.
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.