I created this file to compare markdown editors and test CSS style files for rendering/previewing it. Because it is a testfile, it contains markdown elements that are not rendered by every tool, including GitHub. The purpose is to reveal those elements and compare the look of things in different markdown editors and previewers.
The file can also serve a tutorial and cheat sheet for markdown.
I copied the material from other guides together, in particular the Markdown Guide and the MultiMarkdown V6 Syntax Guide.