Also the venerable https://stackedit.io/.
Also the new GitBook https://docs.gitbook.com/v2-changes/feature-highlights#unifi...
I'm sure there are a few more markdown "WYSIWYM" I'm forgetting...
To build your own see also https://simplemde.com/. Basically CodeMirror is friendly to mixed-font syntax highlighting.
Also yours truly https://mathdown.net