LCCN permalink. A pretty simple thing, but one that makes so much sense. That LC is providing it encourages one to hope such sensical but not quite as simple things may be in the pipeline too.
Also interesting to note that, as far as I’m aware, this means that LC has beat OCLC to having a persistent URI for a (approximated) manifestation that resolves to a publically accessible structured machine actionable bib record (in choice of formats). That LC did it first surely has as much or more to do with business models than it does with tech. It’s actually gratifying and surprising that LC has done it. Stuart Weibel has surely taken note.