calculate the differences between two XML/HTML documents

 Nokogiri::Diff adds the ability to calculate the differences (added or removed
 nodes) between two XML/HTML documents.
 .
 Features
  * Performs a breadth-first comparison between children nodes.
  * Compares XML/HTML Elements, Attributes, Text nodes and DTD nodes.
  * Allows calculating differences between documents, or just enumerating the
  added or removed nodes.