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What is HTML tag?
Does the tag have any attributes?
What is the purpose of using the tag in HTML?
The distinction between <th> and <td> is a semantic one. There are visual differences, but that's just the default user-agent stylesheet, and can be overridden. As part of the html spec, it's specifically intended that <th> headers can be headers for both rows and columns. There's intended to be flexibility in how you use them. It doesn't cause any problems.
In fact, the example in the specification uses a <th> after a <td>.
<tr> <td> <th scope=rowgroup> Cats <td> <td>
http://www.w3.org/TR/2014/REC-html5-20141028/tabular-data.html#the-th-element
TH is supposed to be used as a header cell. If it is a header, then ideally, so should have all the other columns as headers too, coz you should ideally be having a header's row, followed by your data rows. It is a perfectly valid HTML though, but it does not use the correct elements for the purpose.
<table>
<tbody>
<tr>
<th> <a href="">edit</a> <a href="">new</a></th>
<th>I AM THE IMPORTANT CELL</th>
<th>things</th>
<th>more things</th>
</tr>
<tr> <!-- data cells--> </tr>
</tbody>
</table>