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Hypertext links in HTML (draft)

      TOP or ORIGIN
         The TOP relationship identifies the logical top of a
         hierarchical tree of which the current document is a branch.
         BEGIN is a functional equivalent to TOP, if only one  of these
         values is specified.

         When REL=TOP, the target document is the logical top node of
         the tree.  When REV=TOP, the current document is the logical
         top of the tree.

         If the hypertext link is specified with REL in a LINK element,
         an HTML user agent may present an icon in a tool bar.

         NOTE: ORIGIN has been suggested as an alternative to TOP to
         provide metaphorical consistency with PARENT/CHILD/SIBLING.
         Comments are encouraged.

[引用： 4.4d. Hierarchy Links -- Hypertext links in HTML (draft) より]