I use agent a lot for planning, and there is not so much real beautiful and confortable Markdown editor. It gets better but i needed several special things: first-class frontmatter with validation, Sphinx MyST edition, in-cell table edition and do not kill my raw text formatting.
I hope the rendering is extra-clean, allowing to edit a paragraph or a admonition. It supports mermaid, plantuml, vega/vega-lite graph without extern dependencies. It also supports all Sphinx MyST and GitHub Flavored Markdown. I also like the custom "radio choice" similar to GFM tick, really useful for decision making directly in Markdown. I use it as an active working document with my agents and later as a rich collaborative surface in small team. HTML artifacts are amazing with coding agents, but heavy, cannot be displayed as it in Gitlab/Github file, and cannot easily keep states such as choices or micro edition.
It is also used to be a great frontmatter editor OKF Schema that gives a JSONSchema to a frontmatter for OpenKnownledge Format document.