JSFiddle - React, Tailwind, and code Playground

HTML

<form class="formulier" method="post" action="#">
                <input type="text" maxlength="20" name="naam" id="naam">
                <label for="naam">Naam</label><br>
                <div class="naam_error"><br></div><br>
                <input type="url" maxlength="50" name="url" id="url">
                <label for="url">URL</label><br>
                <div class="url_error"><br></div><br>
                <input type="email" maxlength="50" name="email" id="email">
                <label for="email">Email</label><br>
                <div class="email_error"><br></div><br>
                <input type="text" maxlength="50" name="onderwerp" id="onderwerp">
                <label for="onderwerp">Onderwerp</label><br>
                <div class="onderwerp_error"><br></div><br>
                <textarea name="content" id="content" rows="18" cols="50">
Geachte Meneer/mevrouw,

Dit is een test

Ik hoop snel van jullie terug te horen!

Met vriendelijke groeten,

test
                </textarea><br>
                <div class="content_error"><br></div><br>
                <input type="submit" value="verzenden" id="submit_form">
    
            </form>

JavaScript

$("#submit_form").click(function(event){
        //validate process
        var submitnaam = $("input#naam").val();
        var submiturl = $("input#url").val();
        var submitemail = $("input#email").val();
        var submitonderwerp = $("input#onderwerp").val();
        var submitcontent = $("textarea#content").val();
        
        var valid = true;
        //validate realistic length
        
        if(submitnaam.length < 3)
        {
            $('div.naam_error').html('Name is too short.');
            valid = false;
        }
        else
        {
            $('div.naam_error').html('<br>');
        }
        
        if(submiturl.length < 5)
        {
            $('div.url_error').html('URL is too short.');
            valid = false;
        }
        else
        {
            $('div.url_error').html('<br>');
        }
        
        if(submitemail.length < 8)
        {
            $('div.email_error').html('Email is too short.');
            valid = false;
        }
        else
        {
            $('div.email_error').html('<br>');
        }
        
        if(submitonderwerp.length < 3)
        {
            $('div.onderwerp_error').html('Subject is too short.');
            valid = false;
        }
        else
        {
            $('div.onderwerp_error').html('<br>');
        }
        
        if(submitcontent.length < 3)
        {
            $('div.content_error').html('Content is too short.');
            valid = false;
        }
        else
        {
            $('div.content_error').html('<br>');
        }
        if(valid)
        {
            //submit form
            var datastring = 'name='+ submitnaam + '&url=' + submiturl + '&email=' + submitemail + '&onderwerp=' + submitonderwerp + '&content=' + submitcontent;
            $.ajax({
                type: "POST",
                url: "submit.php",
                data: datastring,
                success: function() { 
               ...