This already exists in a mature state - https://mdxjs.com/
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Check it twice, what the submission implements is writing all the UI in markdown, not mixing markdown with React.
Perhaps I don't know mdxjs enough but that's my impression.
MDX parses JSX so it's compatible with any JSX runtime, not just react.
You're right that MDX executes arbitrary code because it supports the `import` and string interpolation of MDX and performs unsafe evals.
Because those evals aren't safe to use at runtime, I actually created a library that uses the MDX parser to support the static subset of JSX (which is very similar to HTML making it static and secure): https://www.npmjs.com/package/react-markdown-with-mdx