I've tried a few SE sites and it seems like all markdown help pages have broken examples. You can see it on the meta se markdown help page or any other SE site. Here is a screenshot from SO:
It should looked like this:
http://web.archive.org/web/20140906093037/https://stackoverflow.com/editing-help
As far as I can tell it's not a CSS problem. I think just some newlines are missing.