Today I've discovered that
<li> elements in list have to be in
<menu> container. It can't be inside
<div> element. Why? It remains a mystery to me (for now).
But there is one huge disadvantage: you have to check it each time manually. Or maybe not?
There is a great program that can check your code and show you errors in terminal, as soon as you save your file. It's called gulp. It makes programmers life way easier, by automatizing repeating actions.