Here's a simple block:

<div>
	foo
</div>

This should be a code block, though:

	<div>
		foo
	</div>

As should this:

	<div>foo</div>

Now, nested:

<div>
	<div>
		<div>
			foo
		</div>
	</div>
</div>

This should just be an HTML comment:

<!-- Comment -->

Multiline:

<!--
Blah
Blah
-->

Code block:

	<!-- Comment -->

Just plain comment, with trailing spaces on the line:

<!-- foo -->   

Code:

	<hr />
	
Hr's:

<hr>

<hr/>

<hr />

<hr>   

<hr/>  

<hr /> 

<hr class="foo" id="bar" />

<hr class="foo" id="bar"/>

<hr class="foo" id="bar" >