If you are serious about your website, you can not just settle with what you see, remember your visitors are worldwide, and they use different screens to access your site – they use even their LCD TV, in 50 inches. The following are must have validations that you should insist your web site developer to check.
1.W3C Markup Validation Service
3.W3C CSS Validation Service
4. Link checker
5. Mobile compliance
Believe this, its worth spending those extra hours and money. !