It would be easier to answer this if we had the specific examples of where the "original HTML" ducuments supposedly recommended this. (I can find an exampl in TBLs axioms, but that was HTML 4 concerning strict and transitional, which seems far from the 'original'.)
As far as I can see, the only case in XML where Postels law really applies to XML would be to provide a (pseudo-)BOM with UTF8: it is unhelpful to generate it, but helpful to accept it on input.
Cheers
Rick