Input:

<em>Emphasis</em>

Result: Emphasis

Input:

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

Result on span:

Result alone:

Without closing:

This is

true

markdown text (paragraph)

This is

true

markdown text (no paragraph)

This is

true

markdown text (block paragraph)

This is

true

markdown text. (no par)

This is

true

markdown text. (par)

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>