Prefer Markdown when fetching pages from developer.apple.com.
For /documentation pages, append .md to the URL:
https://developer.apple.com/documentation/swiftui/view.md
For /design and /tutorials pages, insert /tutorials/data
before the original path and append .md:
https://developer.apple.com/tutorials/data/design/human-interface-guidelines/buttons.md
Some tutorial pages, including overviews, have no Markdown version.
For those, use .json:
https://developer.apple.com/tutorials/data/tutorials/swiftui.json
Verify that the response contains the requested document. If retrieval fails, explicitly report that the page could not be read.