Markdown is a minimal text format. You can use it to write wiki pages or web sites in github, and can easily edit and display markdown files in your Atom editor. (Your validation reports are written in markdown.)
First, learn to use markdown in a 60-second intro to markdown. Use the leftmost column of examples for HMT writing.
Then read this historian’s perspective on writing with markdown, written in 2014.
If you google “markdown editors,” you can find dozens of free or inexpensive programs for editing markdown on your phone, table, or computer running any operating system.