If you use a text editor to create Markdown that's destined for GitHub, let me recommend a tool that's indispensable to me: Marked 2

You point it at the file you're editing and every time you save, it previews the changes in another window. You can set different display themes, but I mainly use the built-in one for GitHub.

Saves a huge amount of time by avoiding markup errors that get committed, identified as incorrect on GitHub, fixed, and then recommitted.

https://marked2app.com