Zettelmarkup
Zettelmarkup is a rich plain-text based markup language for writing zettel content. Besides the zettel content, Zettelmarkup is also used for specifying the title of a zettel, regardless of the syntax of a zettel.
Zettelmarkup supports the longevity of stored notes by providing a syntax that any person can easily read, as well as a computer. Zettelmarkup can be much easier parsed / consumed by a software compared to other markup languages. Writing a parser for Markdown is quite challenging. CommonMark is an attempt to make it simpler by providing a comprehensive specification, combined with an extra chapter to give hints for the implementation. Zettelmarkup follows some simple principles that anybody who knows how ho write software should be able understand to create an implementation.
Zettelmarkup is a markup language on its own. This is in contrast to Markdown, which is basically a super-set of HTML: every HTML document is a valid Markdown document.1 While HTML is a markup language that will probably last for a long time, it cannot be easily translated to other formats, such as PDF, JSON, or LaTeX. Additionally, it is allowed to embed other languages into HTML, such as CSS or even JavaScript. This could create problems with longevity as well as security problems.
Zettelmarkup is a rich markup language, but it focuses on relatively short zettel content. It allows embedding other content, simple tables, quotations, description lists, and images. It provides a broad range of inline formatting, including emphasized, strong, deleted and inserted text. Footnotes2 are supported, links to other zettel and to external material, as well as citation keys. Zettelmarkup allows to include content from other zettel and to embed the result of a search query.
Zettelmarkup might be seen as a proprietary markup language. But if you want to use Markdown/CommonMark and you need support for footnotes or tables, you'll end up with proprietary extensions. However, the Zettelstore supports CommonMark as a zettel syntax, so you can mix both Zettelmarkup zettel and CommonMark zettel in one store to get the best of both worlds.