On the subject of Headered Markdown.
Headered Markdown is a minimal extension to Markdown, or any other possible input format, which adds headers for the document preprocessor to use, that don’t directly make it into the final document. Its use in UmbrellixSSG is to supply control data to UmbrellixSSG that it uses to output suitabletags. Everything after the first empty line in HMD is input to the document processor, which is archetypally Markdown but could be
catif you write your pages in plain HTML.
These headers take a form comparable to email headers.