Upon inspection, it appears that border-collapse is not functioning as expected:
UPDATE: In this case, when referring to border-collapse not working, I am highlighting the issue where the inner table displays a double border, resulting in an unappealing appearance.
A more straightforward version of the code is provided below:
<style>
table, tr, td
{
padding:0;
margin:0;
border-collapse: collapse;
}
td
{
border:1px solid black;
}
</style>
<table cellpadding="0" cellspacing="0">
<tr>
<td>Content 1</td>
<td>Content 2</td>
<td>Content 3</td>
</tr>
<tr>
<td colspan="3">
<table cellpadding="0" cellspacing="0">
<tr>
<td>Content inner</td>
<td>Content inner</td>
</tr>
</table>
</td>
</tr>
<tr>
<td>Content 1</td>
<td>Content 2</td>
<td>Content 3</td>
</tr>
</table>