If it were a WYSIWYG editor, there'd be previews for images. But it seems like it's just syntax highlighting for textareas. Nice project either way, but false advertising.
By definition a “Markdown editor” cannot be WYSIWYG - you can have a WYSIWYG editor that is powered by markdown underneath though, which this is not.
Still a cool project, but someone who does not understand markdown would wonder why pressing the heading button makes my text into a hashtag instead of making it bigger.