The output is not correct, due to a bug in the markdown parser (or misconfiguration).

The snippet:

# This is a header
blablabla

will result in the following HTML:

<p><H1>This is a header></p>
<p>blablabla</p>

This isn't correct, headers shouldn't be inside P blocks. Browsers will move the p block below the header. This causes too much space which is currently compensated in the CSS for header tags (negative margin), but should be corrected in the markdown parser.