Official specification of the AOM group for the carriage of AV1 in ISOBMFF
The specification is written using a special syntax (mixing markup and markdown) to enable generation of cross-references, syntax highlighting, etc. The file using this syntax is index.bs.
index.bs is processed to produce an HTML version (index.html) by a tool called Bikeshed (https://github.com/tabatkins/bikeshed), which is run when content is pushed onto the main branch or when Pull Requests are made.