The CSS for `nature.css` was such that it was preventing `p` tags from
having sufficient vertical whitespace. This meant that you couldn't insert
any kind of spacing between lengthy sections (they just appeared as new lines).
This PR fixes this so you can actually have some whitespace between paragraphs.
As a result of this change, some parts of the spec appeared to have too much
whitespace. These were often sections which shouldn't have begun a new
paragraph anyway (e.g. a single sentence being an entire paragraph, `TODO`
blocks resulting in new paragraphs). This PR fixes the most offending areas
where we shouldn't have been inserting new paragraphs.
