What happens if ignore the HTML5 structure in my web page?

Answer 1

There is only html5, no other structure..

Answer 2

nothing it would try to complete it but dont do that it can cause problems

Answer 3

xml?

Answer 4

If you mean by avoiding html, body, and head elements nothing will go wrong but you will likely have organizational problems and lose default settings, for example, you will have to style your own head instead of using the head element.