Markdown is not a hard format to edit. Any native text editor, paid editor, or even personal knowledge manager can handle Markdown files to some degree. But that is not particularly helpful when the goal is to open a README file in a newly discovered open-source project. Quick Look on macOS, while it displays the file as-is, does not render Markdown properly.

MarkEdit is a free, open-source Markdown editor for Mac that its developers describe as TextEdit dedicated to Markdown. It’s about 4 MB, incredibly lean and fast for the purpose I described. If you choose to open Markdown files with MarkEdit as the default, it also engages Quick Look. It may not pack a punch as the best writing app, but it is a good viewer and editor app.

Like I said in the lede, if your intent lies in writing and editing in Markdown, there are better alternatives. There are a few that come to mind immediately: Ulysses, iA Writer, Lettera (still in closed beta), and so on. If your intent lies in maintaining knowledge or data, and Markdown only happens to be a format of your choice, there are suitable solutions for that purpose as well: DEVONthink, Obsidian, and so on. Many of them are aimed toward professionals, and priced like it. I don’t particularly recommend finding one Markdown editor that rules them all.