What happens if you nest a block element inside an inline element?

Answer 1

NO ,It will not work as the display only changes how the element should be displayed, not the type of the element it is. So, inline element with display:block is not allowed to have other block elements inside it.

Answer 2

It would be valid HTML5 but may not be semantically correct and is often not necessary.

Some justified use may be to make a heading a link:
<a href="#"><h1>A Heading</h1></a>