Define "URL-safe characters" for `research/name.md`

[Ethan-Arrowood]

What are "URL-safe characters"?

• Is this based on the WHATWG URL specification or something else?
• Relevant stackoverflow: https://stackoverflow.com/questions/695438/what-are-the-safe-characters-for-making-urls
• Is the validation for this in the npm cli open-source?