How do I fix a header at the top of a page in CSS?

// When the user scrolls the page, execute myFunction
window.onscroll = function() {myFunction()};

// Get the header
var header = document.getElementById("myHeader");

// Get the offset position of the navbar
var sticky = header.offsetTop;

// Add the sticky class to the header when you reach its scroll position. Remove "sticky" when you leave the scroll position
function myFunction() {
if (window.pageYOffset > sticky) {
header.classList.add("sticky");
} else {
header.classList.remove("sticky");
}
}