---Three hyphens on a line of their own. The rule separates sections that do not deserve a heading of their own.
Careful at the start of a file
Three hyphens at the very top of the file open the frontmatter, not a rule. Anyone wanting to begin a page with a rule puts a paragraph in front of it.
In this template
The rule is a hairline in --tpl-rule with generous space above and below (--tpl-space-xl).
Out of the box Quartz draws a heavier line with less air.