The file markdown.md is the page Markdown of our NicFab Digital Notepad
What is Markdown?
Markdown is a markup language developed by John Gruber in collaboration with Aaron Swartz in 2004.
We quote the definition of Markdown provided by John Gruber:
Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).
Thus, “Markdown” is two things: (1) a plain text formatting syntax; and (2) a software tool, written in Perl, that converts the plain text formatting to HTML. See the Syntax page for details pertaining to Markdown’s formatting syntax. You can try it out, right now, using the online Dingus.
The advantage of Markdown is to write in plain text without worrying about formatting and therefore without distractions since with markup, and you can enrich your text at any time.
We propose publishing the main Markdown commands on this page (continuously updated).
The Markdown logo (The Markdown Mark) 
has been designed and realized by Dustin Curtis (https://dustincurtis.com) with the contribution of Mac Tyler (http://mactyler.com).
The CommonMark project arose because the description of Markdown syntax created by John Gruber is not uniquely specified.
There is a lack of a unique specification of Markdown. That has resulted in different implementations over the years, such that not all editors can reproduce the language uniquely.
For these reasons, John MacFarlane (the developer of Pandoc), Martin Woodward and Jeff Atwood - who call themselves a group of Markdown fans - have decided to create the CommonMark project aimed at defining a single Markdown syntax.
We quote what is on the CommonMark website:
There’s no standard test suite for Markdown; MDTest is the closest thing we have. The only way to resolve Markdown ambiguities and inconsistencies is Babelmark, which compares the output of 20+ implementations of Markdown against each other to see if a consensus emerges.
We propose a standard, unambiguous syntax specification for Markdown, along with a suite of comprehensive tests to validate Markdown implementations against this specification. We believe this is necessary, even essential, for the future of Markdown.
The project is genuinely worthy for those who consider Markdown a relevant resource.
We hope developers, especially those dealing with Markdown editor developments, can refer to the CommonMark specifications to have a unique Markdown syntax.
For Mermaid see this post.