If I have a link inside a.md like

[a](/../../../b.html)

and run lychee with root-dir, then lychee will search for b.html outside of root-dir

$ pwd
/home/x/progs/lychee
$ cargo  run -- a.md --root-dir . --dump
file:///home/b.html

This is probably not a big deal, but it is a difference between --root-dir and being hosted at the root of a website domain.

For instance, a link /../../../b.html from a page at https://example.com/a.html would resolve to https://example.com/b.html. In local files, this would correspond to /home/x/progs/lychee/b.html rather than /home/b.html.