I created the original version of this style guide sometime around 2005, and have been maintaining it ever since. (This past year or two, a colleague has also done a lot of work on it, but I still own it.) We started working on making it publicly available around the beginning of 2016; finally made it visible in June; and have now finally made an announcement about it being available.
The guide itself is probably not useful or relevant to any of you who don't work on Google-related open source documentation projects. But it's a big milestone for me.
Many thanks to all the colleagues who've contributed to making this happen.