HorizontalRule corresponds to <hr> element in the default output
format, HTML. Current HTML standard defines <hr> element as
"paragraph-level thematic break". In typography it is often
represented by extra space or centered asterism ("⁂"), but since
FB2 does not support text centering, empty line (similar to extra space)
is the only solution.
Line breaks, on the other hand, don't generate <empty-line />
anymore. Previously line breaks generated <empty-line /> element
inside paragraph, which is not allowed. So, this commit addresses
issue #2424 ("FB2 produced by pandoc doesn't validate").
FB2 does not have a way to represent line breaks inside paragraphs.
They are replaced with LF character, which is not rendered by
FB2 readers, but at least preserves some information.
<annotation> is not allowed inside <body> according to FictionBook2 XML schema. Besides that, the same information is already placed inside <description>.
Related bug: #2424
