The latest Obsidian release has a WYSIWYG editor mode that's quite good in my experience. It stores everything in plain old markdown files and has a ton of plugins for extra stuff like backlinks, etc.

Take a look at Joplin, it had a WYIWYG editor and normal editor.

It’s like an open source Evernote.

You can try out https://bangle.io, it is a real WYSIWYG Markdown * based note taking app that is local and open source.

* By real I mean that the UI stays WYSIWYG, unlike other apps that fall back to Markdown notation while editing.

I've configured VSCode for this purpose. You can set a proportional font, at which point you are already halfway there. The default syntax highlighting already assigns scopes for headlines, bold/underline/italics, lists, code blocks and links, and it is either easy or default to style the text accordingly.

there's also https://logseq.com/

Dendron has plans to add WYSWG functionality this year

The new Obsidian version does this extremely well.

Obsidian has a "Live Preview" mode.

Obsidian (it's fantastic).