I've been trying to port Chromium's file path <-> file URL utilities to a project conforming to the latest standard.

As far as I have been able to tell (it's a large codebase and I'm not at all familiar with it), Chromium turns Windows UNC paths in to file URLs with hostnames, and those hostnames may include percent-encoding (e.g. \\some computer\foo\bar.txtbecomesfile://some%20computer/foo/bar.txt).

That is not allowed by this standard: the hostnames of file URLs are domains (they are even encoded with IDNA), and may not contain raw spaces or percent-encoding.

I don't think it is expected that a file URL's host must only be a domain or IP address; it might be better for them to have opaque hostnames, so they can contain percent-encoding and other characters as allowed by whatever mechanism resolves them.