Section 2.6.1 defines a hierarchical URL thus:

"An absolute URL is a hierarchical URL if, when resolved and then parsed, there is a character immediately after the <scheme> component and it is a U+002F SOLIDUS character (/)."

However, RFC3986 Section 3 defines all URIs as containing a hierarchical part as follows:

URI         = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

and, further, does not require the hierarchical part to start with "/". In particular, it defines hier-part as:

hier-part   = "//" authority path-abempty
                  / path-absolute
                  / path-rootless
                  / path-empty

Which, when expanding these components into their definitions, corresponds to:

hier-part
          = "//" authority
          | "//" authority 1*( "/" segment )
          | "/" [ segment-nz *( "/" segment ) ]
          | segment-nz *( "/" segment )
          | 0<pchar>

Note that the last two alternatives do not start with "/", yet are still considered a "hierarchical" part by RFC3986. For example, the following URIs match this syntax, with hier-part mapping to path-rootless:

about:blank
file:foo/bar
urn:example.net:foo:bar

In order to avoid confusion, it may be desirable to use a different term in HTML5 than "hierarchical URL" in this regard. Alternatively, a note could be added which distinguishes the defined usage from the like named (but different) constructs in RFC3986.

I would also note that, in terms of the definitions found in 2.6.1, all "authority-based URLs" are also "hierarchical URLs". I can't tell if this is intentional or not, if it is, then perhaps a note indicating this would be useful.

Regards,
Glenn