JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>

  <head>

    <script>
      function allowDrop(ev) {
        ev.preventDefault();
      }

      function drag(ev) {
        ev.dataTransfer.setData("text", ev.target.id);
      }

      function drop(ev) {
        ev.preventDefault();
        var data = ev.dataTransfer.getData("text");
        ev.target.appendChild(document.getElementById(data));
      }

    </script>


    jQuery(function($) {
        var variable_types = {
            'Germany': ['Duesseldorf', 'Leinfelden-Echterdingen', 'Eschborn'],
            'Spain': ['Barcelona'],
            'Hungary': ['Pecs'],
            'USA': ['Downers Grove'],
            'Mexico': ['Puebla'],
            'South Africa': ['Midrand'],
            'China': ['Beijing'],
            'Russia': ['St. Petersburg'],
        }

        var $variable_types = $('#location');
        $('#country').change(function () {
            var country = $(this).val(), lcns = variable_types[country] || [];

            var html = $.map(lcns, function(lcn){
                return '<option value="' + lcn + '">' + lcn + '</option>'
            }).join('');
            $variable_types.html(html)
        });
    });


    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
  </head>

  <body>



    <form id="myForm">
      <input type="button" id="NewAxe" value="Ajouter un axe">
      <input type="text" id="NewAxeName" value=""><br>
      <input type="button" id="DelAxe" value="Supprimer le dernier axe">
    </form>


    <div style="width: 100%; display: table;">
      <div id="multi_axe_container" style="display: table-row">
        <div id="multi_variable_container" class="axe_container" ondrop="drop(event)" ondragover="allowDrop(event)">
          <div id="var0" class="axe_title">Variables disponibles : </div>
          <div id="var1" class="axe_title" draggable="true" ondragstart="drag(event)">Première variable
            <br />

            <label...

CSS

/* Layout */

body {
  background-color: rgb(230, 230, 230);
  margin: 30px;
  padding: 20px;
}

.background {
  background-color: rgb(255, 255, 255);
  margin-top: 30px;
  margin-bottom: 30px;
  margin-left: 90px;
  margin-right: 90px;
  padding-top: 30px;
  padding-bottom: 30px;
  padding-left: 60px;
  padding-right: 60px;
  box-shadow: 0px 0px 8px rgba(100, 100, 100, 0.5);
  border-radius: 2px;
}

.center {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

/* Titles */

h1 {
  font-size: 3em;
  font-variant-caps: small-caps;
  font-weight: bolder;
  color: rgb(0, 106, 77);
  vertical-align: middle;
  text-align: left;
}

h2 {
  font-size: 2em;
  font-weight: bold;
  color: rgb(65, 65, 65);
}

h3 {
  font-size: 1.7em;
  font-weight: bold;
  color: rgb(0, 136, 144);
}

h4 {
  font-size: 1.5em;
  font-weight: bold;
  color: rgb(68, 68, 68);
}

h5 {
  font-size: 1.3em;
  font-weight: bold;
  color: rgb(98, 98, 98);
}

p {
  font-size: 1.1em;
  color: rgb(117, 117, 117);
  text-align: justify;
}

.username {
  font-variant-caps: small-caps;
  font-weight: bold;
}

.error {
  font-size: 1.1em;
  font-style: italic;
  color: rgb(187, 15, 23);
  text-align: center;
  margin-bottom: 10px;
}

/* Header */

header {
  display: flex;
  flex-direction: row;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  margin-left: 20px;
  margin-right: 40px;
}

.title {
  display: flex;
  align-items: center;
  justify-content: flex-start;
  flex-wrap: wrap;
}

.blank {
  color: rgb(255, 255, 255);
}

.tabs {
  width: 97%;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: 40px;
  margin-top: 20px;
  padding: 10px;
  display: flex;
  justify-content: flex-start;
  align-items: center;
  flex-wrap: wrap;
  border-bottom-style: solid;
  border-top-style: solid;
  border-color: rgb(230, 230, 230);
  color: rgb(117, 163, 149)
}

.form {
  border-bottom-style: solid;
  border-top-style: solid;
  border-color: rgb(230, 230, 230);
  color:...