Insure Landing Page Design with pure HTML, CSS and JS

The area I think I'm unsure of is the nav. When the viewport is changed in the browser from a smaller screen to a bigger one, the desktop navbars do not show up unless you reload. This isn't really a problem since most people won't try to see your page on their mobile phones and change the orientation.

I would like some feedback on generally every aspect of my code though.

Thanks.