Citation Style Language (CSL) is simply a very important project for publishing. It has widespread adoption in important platforms and plays a critical role in the scholarly publishing landscape. It is also important in that it is an open source project populated by a diverse set of skill sets and research perspectives.
The Citation Style Language project developed an XML-based format to define citation formats. Originally it was built to support the OpenOffice platform but has since been adopted on a wide scale.
The projects maintains a free and open source repository with currently over 9000 CSL citation styles for major style guides and individual journals (see https://github.com/citation-style-language/styles/ and https://pinux.info/csls_counter/). Dozens of software products, including popular reference managers such as Zotero, Mendeley, and Papers, have adopted CSL and its style library to give their users the ability to easily generate citations in a large variety of citation formats.