http://url.spec.whatwg.org/#relative-path-start-state

[[
relative path start state
1. If c is "\", parse error.

2. Set state to relative path state and if c is neither "/" nor "\", decrease pointer by one.

relative path state
1. If either c is one of EOF code point, "/", and "\", or state override is not given and c is one of "?" and "#", run these substeps:

  1. If buffer is ".." and c is one of EOF code point, "/", and "\", pop url's path, if non-empty, and then append the empty string to it.

  2. Otherwise, if buffer is "..", pop url's path, if non-empty.

  3. Otherwise, if buffer is "." and c is one of EOF code point, "/", and "\", append an empty string to url's path.

  4. Otherwise, if buffer is not ".", run these subsubsteps:
]]

I suggest the following changes to that text:

-----------------------
relative path start state

2. Set state to relative path state and decrease pointer by one.
-----
relative path state

1. [...]

   1. If buffer is "..", pop url's path, if non-empty, then if c 
       is neither "/" nor "\", append the empty string to url's path.

   2.  Otherwise, if buffer is "." and c is neither "/", nor "\", append an 
        empty string to url's path.

   3.  Otherwise, if buffer is neither ".." nor ".", run these subsubsteps [...]
-----------------------

With these changes, I believe the following test cases will pass:
   
/x/w/../y -> /x/y
/x/w/.. -> /x/
/x/w/..#z -> /x/#z

////../.. -> //
/x/x/x/../.. -> /x/

/x/../y -> /y
/x/.. -> /
/x/..#z -> /#z

/../y -> /y
/.. -> /
/..#z -> /#z

/x/./y -> /x/y
/x/. -> /x/
/x/.#z -> /x/#z
/./y -> /y
/. -> /
/.#z -> /#z