nav element slides onto screen after scroll past header.
A short script in vanilla JS.
Target navigation should be in a nav tag, and the first element of the page should have a header tag.
The navigation is hidden until the user scrolls past the bottom of the header, whereupon it slides into view.
And vice-versa.
I recommend adding some CSS in order to prevent a flash of the nav on pageload:
nav{
visibility: hidden;
position: fixed; }