This may work for very simple cases:
<layout type="log4net.Layout.PatternLayout">
<conversionPattern>
<![[CDATA[
<table>
<tr>
<td><strong>%level</strong></td>
<td>%date</td>
<td>%message</td>
</tr>
</table>
]]>
</conversionPattern>
</layout>
However Log4Net complains about this method of specifying a pattern - seems the pattern can't be defined in the inner html; that it must be in the value attribute of the <conversionPattern> tag. (Also I presume the <