Is rel single or bi-directional?
Though the HTML4 specification is not specific on this point, all the rel values in the HTML4 specification follow a very consistent pattern that indicate a specific directionality to the value of the rel attribute. The only exception to this is the value “alternate”, which, is actually inherently symmetric, so rel=”alternate” means pretty much the same thing as rev=”alternate”. I say pretty much because in theory (and in practice) one of a pair of alternates is likely to be more definitive, and thus one is more a primary resource, with the other being an alternative. In theory you could extend rel with a new value, e.g. “original” which could be used to point back the original version of a document. This could be used for example with translations, where translations of a document would point back to the original with rel=”original”.
