onFOSS is a collective idea of an online, FOSS lan-pary-kind event hosted by different people found here: onFOSS
The onFOSS event was originally created by hribhrib, but it got a whole rebranding and structuring in 2023 and moved all together to the new domain onFOSS.org where all information is there. There is also a blog-post about the changes here: onFOSS: The next generation!. This website "onfoss.libregaming.org" is now just here for historic reasons.
The sourcecode of the website can be found here