Responsive Intro section with dropdown navigation using mobile first

I think your code could be improved by reducing the redundancy. A lot of the elements are selected using specific classes despite having the same functions and events. To solve this, instead of:

const elementOne = document.querySelector(".element1");
const elementTwo = document.querySelector(".element2");
...
const elementN = document.querySelector(".elementN");

give them a unifying class name that can call them all simultaneously:

const elements = document.querySelectorAll(".element");

Then you can write a function that handles the conditional logic and pass it to the array of elements so you aren't writing the same/similar conditionals repeatedly:

function handleEl(el, optionalEl) {
...
}

elements.forEach(element => {
element.addEventListener('click', () => handleEl(element))
});

You could improve on the hover states as well. As it is now, your css has no :hover on the links and buttons (the font color is supposed to get darker.) Refer to the style-guide.md.