Hopefully this guide will be updated as time goes on to stay relevant as technology changes. Otherwise in a few years from now, everything will look outdated again.
What's more interesting is if this will lead to better maintainability. Since this includes code as well as guidelines, hopefully it will reduce the variation between various agencies' websites. That in turn should make future upgrades more uniform and therefore less costly.
[1] http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3810021...
https://www.gov.uk/government/publications/traffic-signs-man... (chapter 7 is best, for kerning rules, and making and face the right way).
They're also pretty active with the pull request system: https://github.com/18F/web-design-standards/pulls and have a CONTRIBUTING guide: https://github.com/18F/web-design-standards/blob/18f-pages-s...
I attended a presentation by one of the people working on the project last week and they put a lot of thought into both how to make this a usable guide for now and how to keep it growing into the future. The USDS has attracted some really great people lately, and it shows!