Below is a table.
<table border="50%">
<tr>
<th>Hello</th>
<th>World!</th>
</tr>
<tr>
<td>Hello</td>
<td>Hello</td>
</tr>
<tr>
<td>Hello</td>
<td>Hello</td>
</tr>
</table>
How can border be given just 50% and nothing else as a value? What is the black area surrounding the table? Why is one side of the black area darker than the other?