Well, it's not OpenSource, nor Markdown, but free, private and perfect to use ro design a Website fast and easy. It's the Blue Velvet Editor, a complete wordprocessor or Html editor, which save the files locally in Html format by default, so it only need to upload it as is to an host. You can even select the content of an website and paste it in the editor, there it will appear in its original UI with all working links. Works in PC and Mobile, in the browser or as PWA.