Beside of any text editor like Sublime Text, Visual Studio Code which can edit markdown files easily, but lack WYSIWYG, there are some other apps to do this task
An in-browser markdown editor, with sync scrolling, mermaid diagram support, katex, publish online to GitHub or html, import/export features, WYSIWYG.
An Electron app, with WYSIWYG. But slow 1st open time, and heavy (500 MB install folder size).
This is a powerful yet simple note taking web app that works on top of your locally saved Markdown files.
Support backlink, tags, folder view, search.
It is a competitor of StackEdit