It's not an editor, but ProseMirror is something that I'd recommend checking out if you wanted to build a web-based WYSIWYG markdown editor with possibly extend

It's not an editor, but ProseMirror is something that I'd recommend checking out if you wanted to build a web-based WYSIWYG markdown editor with possibly extended features. There's also Notion.so. Both of these aren't explicitly markdown, but if you keep the representation simple and flat you can always export it as such. I came across both of these editors when I saw a friend with a Chrome extension (Papier) which replaced new tabs with a markdown-esque page, but it didn't support inline math. Marktext, as mentioned in other comments, is open-source but I found it hard to just take the editor Muya and attach that onto a webpage.