The element inside the tag <td>is the text node. You cannot address it in CSS. If you want to style it, you will need to tag the tag <td>or surround the text you want to style with the element <span>.
To, for example, stylize only Hello:
<table>
<tr>
<td><span>Hello<span> StackOverFlow</td>
</tr>
</table>
And in your CSS
span { font-weight: bold; }
Or any style you want to give him.
. .