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" >