<table style="border-collapse: separate; border-spacing: 0; background: #baffc9"><tbody>
	<tr id="tr" style="margin: 0; border: 10px solid black; padding: 0">
		<td style="margin: 0; border: 0; padding: 0; height: 42px; width: 42px;"></td>
	</tr>
</tbody></table>
    <div id="result"></div>
<script>
var result = document.getElementById('result');
var tr = document.getElementById('tr');
result.innerHTML = getComputedStyle(tr).width;
</script>