Input:

<em>Emphasis</em>

Result:

Input:

<img src="http://jigsaw.w3.org/css-validator/images/vcss"/>

Result on span:

Result alone:

Without closing:

The following is invalid HTML, and will generate an error:

HTML parse error: 
<table>
<td markdown="1">This is *true* markdown text. (no par)</td>
<td markdown="block">This is *true* markdown text. (par)</td>
</tr>

HTML parse error: 
</table>