JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id="form">
        
<h3>Aanmelding WIES Congres</h3>

        <form name="myForm" action="showform.php" onsubmit="return validateForm();" method="post">
            <INPUT TYPE="hidden" NAME="to" VALUE="a" />
            <p class="legend">Deelnemer</p>
            <fieldset id="deelnemer">
                <label>Naam</label>
                <input type="text" name="Naam" size="30" />
                <label>Bedrijf</label>
                <input type="text" name="Bedrijf" size="30" />
                </br>
                <label>Adres</label>
                <input type="text" name="Adres" size="30" />
                <br/>
                <label>Postcode</label>
                <input type="text" name="Postcode" id="postcode" onblur="myPostcode();" size="30" />
                <label>Plaats</label>
                <input type="text" name="Plaats" size="30" />
                </br>
                <label>Land</label>
                <select name="land">
                    <option value="Nederland" selected="selected">Nederland</option>
                    <option value="Belgie">Belgie</option>
                    <option value="Duitsland">Duitsland</option>
                    <option value="Frankrijk">Frankrijk</option>
                    <option value="Luxemburg">Luxemburg</option>
                </select>
                <br/>
                <br/>
                <label>WIES ID</label>
                <input type="radio" name="Ja" value="Ja" id="Ja" />Ja
                <input type="radio" name="Ja" value="Nee" id="Nee" />Nee
                <br/>
                <label class="lidnummer">Lidnummer</label>
                <input type="text" name="getal1" maxlength="6" size="25" />
                <br/>
                <label class="lidnummer">WIES pin</label>
                <input type="password" maxlength="4" name="getal2" size="25" />
                <br/>
                <div>
<span id="NaamVerplicht" class="Error">* Naam is...

JavaScript

function validateForm() {
    var errorFound = false;

    if (isEmpty("Naam")) {
        errorFound = true;
    }

    if (isEmpty("Adres")) {
        errorFound = true;
    }

    if (isEmpty("Postcode")) {
        errorFound = true;
    }

    if (isEmpty("Plaats")) {
        errorFound = true;
    }
    
    if (!document.getElementById("radio1").checked &&         !document.getElementById("radio2").checked&&         !document.getElementById("radio3").checked) {
        errorFound = true;
    }
    
    if (!document.getElementById("radio4").checked &&         !document.getElementById("radio5").checked&&         !document.getElementById("radio6").checked) {
        errorFound = true;
    }

    return (!errorFound)



}





function isEmpty(fieldName) {
    var x = document.forms["myForm"][fieldName].value;

    if (x == null || x == "") {
        document.forms["myForm"][fieldName].className = "required";
        document.getElementById(fieldName + "Verplicht").style.display = "inline";

        return true;
    } else {
        document.forms["myForm"][fieldName].className = "";
        document.getElementById(fieldName + "Verplicht").style.display = "none";

        return false;
    };
}

function myPostcode() {
    var x = document.getElementById("postcode").value;
    document.getElementById("postcode").value = x.toUpperCase();
}