The `raw_attribute` will be used to mark raw bits, even HTML
and LaTeX, and even when `raw_html` and `raw_tex` are enabled,
as they are by default.
To get the old behavior, disable `raw_attribute` in the writer.
Closes#4311.
This improves on the original fix to #5285 by preventing
other mixed lists (lists with a mix of Plain and Para
elements) that were allowed given the original fix.
This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.