First child has no vertical margin

A comment like this

First paragraph.

    Some preformatted text.

Second paragraph.

looks bad: the vertical margin around the preformatted text is uneven. Specifically, there is no margin between the first paragraph and the code block, but there is a margin between the block and the second paragraph.

The culprit is this rule in your CSS:

.wiki-content > *:first-child {
  margin-top: 0;
  margin-bottom: 0;
}

Maybe you could just reset the top margin?