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.

WWW: https://github.com/postmodern/nokogiri-diff