As part of a location path, the only axes for which support is required are:
child (in abbreviated syntax this is the default axis, represented simply by /, the location path separator).
descendant-or-self (abbreviated syntax: //).
attribute (abbreviated syntax: @).
Support for the other axes is not required.
See 6.6.3.3 Property Constraint and 6.6.3.4 Path Constraint, above.