What we mean by the visual and non visual part in html??

Answer 1

visual part - body
non visual part - head

Answer 2

the Visual : front-end is all about what you see like the bouton the background but the non visual part as you say is exemple when you'll click on the button what it will do ? or when you are login to a site the backend is checking with the database to see if your info are right this is the "non visual part"