No one is perfect all are being to be perfect so for the beginer designer they always think about why to  validate  the CSS. Is it necessary or not. They think why to validate the webpage if they are showing correctly. But it is very necessary to validate the website because  advanced css coders can get away with validating their work less frequently during development but for beginners it is a must and should be done frequently. In this way, silly errors are avoided and the beginner is also alerted to the fact that they may be using an incorrect property or value.

It’s basically like using a spell checker which will fix your words but doesn’t mean that you can write anything sensible. You still need common sense and to have some idea of what you are doing.