RFC 3986 defines Uniform Resource Identifiers.
A relative reference that begins with a single slash character is
termed an absolute-path reference. A relative reference that does
not begin with a slash character is termed a relative-path reference.
href="foo/bar.html"
is a relative reference, specifically a relative-path reference.href="/foo/bar.html"
is a relative reference with an absolute pathhref="http://example.com/foo/bar.html"
is an absolute URIhref="//example.com/foo/bar.html"
is a network-path reference
A relative reference that begins with two slash characters is termed a network-path reference; such references are rarely used.