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A basic implementation here: http://webpass.rkeene.org/

A couple of difficulties your writeup doesn't cover:

* password expiry/rotation - webpass uses the "sequence number" for this, and sequence numbers can be saved in localStorage or shared online through another channel

* policies. The "Site" dropdown references a json structure which can specify min/max lengths, character type rules etc

The implementation is pretty simple, but that's kinda the point :).