Hacker News Reader

The software is proprietary, but the actual files use a variant of an open ISO/IEC/ECMA standard.[1]

[1] https://en.wikipedia.org/wiki/Office_Open_XML

IIRC, it is essentially impossible to correctly implement parts of OOXML without reverse-engineering the Office suite. The standard does not provide enough information.

Reply on news.ycombinator.com