It's been kind of assumed that anything that starts with / is an absolute rather than relative URL, because you're claiming the dir hierarchy from the root of (wherever your web pages are). If you start with / then the domain is assumed. On the other hand, if you start with anything OTHER than / then it is certainly a relative link.
It's like the difference between these 3 statements:
rm /adir/another/file.txt -- remove file.txt in (root)/adir/another
rm ../adir/file.txt -- remove file.txt that is one a directory one up from me then into adir
rm adir/file.txt -- remove file.txt which is in the directory adir below me
URLs work exactly the same way. This immediately looks to be cool. It is not, because it makes page position in a website dependent on current position. It's particularly troublesome for spiders and scrapers. Net-net: I'd not use relative URLs unless it was absolutely necessary. And I've never found it absolutely necessary