Both the original and CommonMark specs use the term “horizontal rule” to describe Markdown syntax that produces an HTML <hr />
tag. However the HTML5 spec makes no mention of horizontal rules. Instead, the term “thematic break” is used to describe the meaning of the <hr>
element:
The hr element represents a paragraph-level thematic break, e.g. a scene change in a story, or a transition to another topic within a section of a reference book.
The meaning has changed since HTML4, where <hr>
did represent a horizontal rule:
The HR element causes a horizontal rule to be rendered by visual user agents.
The meaning was likely changed to make <hr>
have semantic rather than presentational meaning, the same way that <i>
and <b>
were redefined.
To stay consistent with HTML5 it would be worth changing the terminology in the spec to “thematic break”. Also, the semantic meaning is more accurate; websites may style <hr>
elements as images rather than horizontal rules, for example.