I think I've got a pretty good grasp of CSS principles at an intermediate level, but today I saw something that I can't figure out.

I'm studying a page (http://www.nodebeginner.org/) whose style sheet defines rules like this:

Code:
p {
   margin-bottom: 48px;
   margin-top: -22px;
}

pre {
   margin-bottom: 64px;
   margin-top: -24px;
}

ul {
   margin-bottom: 64px;
   margin-top: -24px;
}

ul li {
   margin-bottom: 12px;
}

h2 {
   margin-bottom: 36px;
   margin-top: 64px;
}

h3 {
   margin-bottom: 36px;
   margin-top: 36px;
}
I think I understand what the rules are doing, but not why. What's the purpose of defining negative margins?