I have something similar to that:
<table>
<tr>
<td>Hello,<br/>World!</td>
</tr>
</table>
Both lines
Hello,
World!
are displayed too close to one another. Any way to increase spacing between them (by a portion of a line width (without another <br/>))?
Use line-height to adjust the, well, line height. So in your case line-height: 2 will double the line height.
Here's a full example of how to make the line spacing within <td>s one and a half times the height of the font:
<html><head>
<style>
td {
line-height: 150%;
}
</style>
</head>
<body>
<table>
<tr>
<td>Hello,<br/>World!</tr>
</tr>
</table>
</body></html>