To embed an HTML app inside of a document, use the app's folder-local .html path with an iframe block:
<iframe src="./file-index.html"></iframe>
The src must be a relative folder-local .html path from the Markdown File.
Inline Embeds resize to fit their content inside the Markdown File. A full-panel HTML App opened on its own relies on normal iframe scrolling instead.
To author the HTML App itself, see create-html-app.