2nd Edition (2003)

This release is the Portable Network Graphics (PNG) Specification (Second Edition)
Information technology — Computer graphics and image processing — Portable Network Graphics (PNG): Functional specification. ISO/IEC 15948:2003 (E)
W3C Recommendation 10 November 2003

It can be found at https://www.w3.org/TR/2003/REC-PNG-20031110/