How Html5 Page Structure Affects W3c Validation And Seo

If we declare page as HTML5, Is it mandatory to follow HTML5 page structure ? . Below are two examples shows the ideal HTML5 page and page which is not following HTML5 structure.

Solution 1:

1. Yes. But: those <header>, <nav> etc elements are not mandatory! A HTML5 document is a valid HTML5 document even if they are "missing".
2. No.
3. I beg your pardon?
4. Can you give an example? If you mean proprietary style properties like -webkit-appearance, yes, you'll have to live with the fact that those don't validate.
5. I'm not sure they do, actually. They may have an effect in the future though.