HolySong,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
tbody td:hover:after {
content: attr(data-th);
color: #FF0000;
}
</style>
<script>
document.addEventListener("DOMContentLoaded", function() {
let table = document.querySelector('table');
let txt = Array.from(table.querySelectorAll('thead td'), ({
textContent
}) => textContent);
let len = txt.length;
table.querySelectorAll('tbody td').forEach((td, i) => td.dataset.th = txt[i % len])
});
</script>
</head>
<body>
<table>
<thead>
<tr>
<td>Заголовок 1</td>
<td>Заголовок 2</td>
<td>Заголовок 3</td>
</tr>
</thead>
<tbody>
<tr>
<td>Ячейка 4</td>
<td>Ячейка 5</td>
<td>Ячейка 6</td>
</tr>
<tr>
<td>Ячейка 7</td>
<td>Ячейка 8</td>
<td>Ячейка 9</td>
</tr>
</tbody>
</table>
</body>
</html>