Chess Programming Wiki
https://www.chessprogramming.org
Chess Variant Pages
https://www.chessvariants.com
Elo rating system
https://en.wikipedia.org/wiki/Elo_rating_system
Standard: Portable Game Notation Specification and Implementation Guide
https://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm
Stockfish Documentation
https://official-stockfish.github.io/docs
Talk Chess
https://talkchess.com/