Fixed Header on Y

How to fix the header section only on the Y axis to allow x-scrolling?

HTML

<table class="tg">
  <tr>
    <th class="tg-i7wz">HEADER 1</th>
    <th class="tg-031e">HEADER 2</th>
    <th class="tg-031e">HEADER 3</th>
    <th class="tg-031e">HEADER 4</th>
    <th class="tg-031e">HEADER 5</th>
    <th class="tg-031e">HEADER 6</th>
    <th class="tg-031e">HEADER 7</th>
    <th class="tg-031e">HEADER 8</th>
    <th class="tg-031e">HEADER 9</th>
    <th class="tg-031e">HEADER 10</th>
    <th class="tg-031e">HEADER 11</th>
    <th class="tg-031e">HEADER 12</th>
    <th class="tg-031e">HEADER 13</th>
    <th class="tg-031e">HEADER 14</th>
    <th class="tg-031e">HEADER 15</th>
    <th class="tg-031e">HEADER 16</th>
    <th class="tg-031e">HEADER 17</th>
    <th class="tg-031e">HEADER 18</th>
    <th class="tg-031e">HEADER 19</th>
    <th class="tg-031e">HEADER 20</th>
  </tr>
  <tr>
    <td class="tg-031e">CONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENT </td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
  </tr>
  <tr>
    <td class="tg-031e">CONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENTCONTENT CONTENT</td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td class="tg-031e"></td>
    <td...

CSS

.header {
    width: 2000px;
}
.tg  {
    border-collapse:collapse;
    border-spacing:0;
}
.tg td {
    font-family:Arial, sans-serif;
    font-size:14px;
    padding:10px 5px;
    border-style:solid;
    border-width:1px;
    overflow:hidden;
    word-break:normal;
}
.tg th {
    font-family:Arial, sans-serif;
    font-size:14px;
    font-weight:normal;
    padding:10px 5px;
    border-style:solid;
    border-width:1px;
    overflow:hidden;
    word-break:normal;
}
.tg .tg-i7wz {
    font-size:20px
}