JSFiddle - React, Tailwind, and code Playground

HTML

<form id="abo" method="post">
    <fieldset>
        <legend>Jeg vil:</legend>
        <label>
            <input name="jegvil" required="" type="radio" checked="" value="bestille et abonnement">bestille et abonnement</input>
        </label>
        <br/>
        <label>
            <input name="jegvil" required="" type="radio" value="kjøpe kun siste nummer">kjøpe kun siste nummer</input>
        </label>
    </fieldset>
    <fieldset>
        <legend>Informasjon om deg</legend>
        <label for="a">Fornavn:</label>
        <input name="fornavn" id="a" required="" type="text" placeholder="Ola"></input>
        <br/>
        <label for="b">Etternavn:</label>
        <input name="etternavn" id="b" required="" type="text" placeholder="Nordmann"></input>
        <br/>
        <label for="c">E-postadresse:</label>
        <input name="mail" id="c" required="" type="email" placeholder="[email protected]"></input>
        <br/>
        <label for="d">Telefonnummer:</label>
        <input name="tel" id="d" required="" type="tel" placeholder="12345678" minlength="8"></input>
        <br/>
        <label for="e">Gateadresse:</label>
        <input name="gate" id="e" required="" type="text" placeholder="Liljevegen 3"></input>
        <br/>
        <label for="f">Postnummer:</label>
        <input name="postnummer" id="f" required="" type="number" maxlength="4" placeholder="8400" minlength="4"></input>
    </fieldset>
    <fieldset>
        <legend>Hvordan fikk du høre om magasinet?</legend>
        <label>
            <input name="innflytelse[]" type="checkbox" value="På dansegalla/dansefest">På dansegalla/dansefest</input>
        </label>
        <br/>
        <label>
            <input name="innflytelse[]" type="checkbox" value="Annonse på nett">Annonse på nett</input>
        </label>
        <br/>
        <label>
            <input name="innflytelse[]" type="checkbox" value="Via Facebook">Via Facebook</input>
        </label>
        <br/>
        <label>
           ...

JavaScript

document.getElementById('abo').addEventListener("change", function () {
	console.log('changed');
    if (document.querySelector("form fieldset:first-child input[value='bestille et abonnement']").checked) {
       	console.log('first');
        document.getElementById("hardukode").removeAttribute("style");
        document.getElementById("tskjorte").removeAttribute("style");
        if (document.querySelector("#hardukode input[value='ja']").checked) {
            var el = document.querySelectorAll("#hardukode label:last-of-type, #hardukode br, #hardukode > input:last-of-type");
            for (i = 0; i < el.length; i++) {
                el[i].removeAttribute("style");
            }
            document.getElementById("tskjorte").style.display = "none";
        } 
        	else {
            var el = document.querySelectorAll("#hardukode label:last-of-type, #hardukode br, #hardukode > input:last-of-type");
            for (i = 0; i < el.length; i++) {
                el[i].style.display = "none";
            }
            document.getElementById("tskjorte").removeAttribute("style");
        }
        if (document.querySelector("#tskjorte input[value='ja']").checked) {
            var el = document.querySelectorAll("#tskjorte #i, #tskjorte #j, #tskjorte label[for='i'], #tskjorte label[for='j'], #tskjorte br")
            for (i = 0; i < el.length; i++) {
                el[i].removeAttribute("style");
            }
        } 
        	else {
            var el = document.querySelectorAll("#tskjorte #i, #tskjorte #j, #tskjorte label[for='i'], #tskjorte label[for='j'], #tskjorte br");
            for (i = 0; i < el.length; i++) {
                el[i].style.display = "none";
            }
        }
    
    } 
     else {
	    console.log('!first');
        document.getElementById("tskjorte").style.display = "none";
        document.getElementById("hardukode").style.display = "none";
    }

    if (document.querySelector("#likerdudanse input[value='ja']").checked ||...