.header{
width: 100%;
height: 80px;
transition-duration: 3s;
z-index: 2;
position: fixed;
background: rgba(0, 0, 0, 0);
}
<div class="content">
<div class="default">Див, прописанный в разметке</div>
<div class="custom">Див, добавленный скриптом</div>
</div>
<script>
document.addEventListener('DOMContentLoaded', function() {
let custom = document.querySelector('.custom')
console.log(custom);
})
</script><!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<div>
<p><input type="radio" name="radio-btn" id="" class="radio radio1">Радио 1</p>
<p><input type="radio" name="radio-btn" id="" class="radio radio1">Радио 1</p>
<p><input type="radio" name="radio-btn" id="" class="radio radio2">Радио 2</p>
<p><input type="radio" name="radio-btn" id="" class="radio radio2">Радио 2</p>
<p><input type="radio" name="radio-btn" id="" class="radio radio2">Радио 2</p>
<div class="content1 cont" >content 1</div>
<div class="content2 cont">content 2</div>
</div>
</body>
<style>
.cont{
display: none;
}
</style>
<script>
let cont1 = document.querySelector(".content1")
let cont2 = document.querySelector(".content2")
let radios1 = Array.from(document.querySelectorAll(".radio1"))
let radios2 = Array.from(document.querySelectorAll(".radio2"))
radios1.forEach(function(r1){
r1.addEventListener("input", function(){
cont2.style.display = "none"
cont1.style.display = "unset"
})
})
radios2.forEach(function(r2){
r2.addEventListener("input", function(){
cont1.style.display = "none"
cont2.style.display = "unset"
})
})
</script>
</html>