IMO, there's no such thing as a WYSIWYG Markdown editor. Markdown is a plain text format that defines neither presentation logic nor formatting instructions. Markdown editors typically convert documents to HTML and then display the HTML with CSS to adjust the look and feel.
KeenWrite is what-you-see-is-what-you-mean editor, similar to LyX. The preview panel shows the content having been converted to XHTML and formatted with CSS. When producing a PDF file, the XHTML is sent through the ConTeXt typesetting software and formatted according to the instructions associated with a user-defined (or user-selected) theme. While the XHTML preview is rendered in real-time, the typesetting is not real-time.
Well, Markdown as a format is intended to be viewed rendered as HTML (or equivalent), and WYSIWYG means that the editing takes place in the rendered representation. Markdown is only a storage format then, that merely happens to also be human-readable. That the is rendered representation itself is not fixed and can be styled or themed independently is not a contradiction IMO. But call it what you want.
The main reason I’m interested in WYSIWYG editing for Markdown is that I find monospaced fonts to be too hard on the eyes for editing prose text of any length. But due to lists and code fragments and the like, just switching to a variable-width font in a plain-text editor is also not very practical.
KeenWrite is what-you-see-is-what-you-mean editor, similar to LyX. The preview panel shows the content having been converted to XHTML and formatted with CSS. When producing a PDF file, the XHTML is sent through the ConTeXt typesetting software and formatted according to the instructions associated with a user-defined (or user-selected) theme. While the XHTML preview is rendered in real-time, the typesetting is not real-time.