Here you can download the Active CSS documentation as an offline SPA. You can do this if you want to keep a specific version of the website for reference purposes.
It is a fully navigable offline version of the website, including the code editor and the search functionality. The only things that won't work are the examples that use ajax requests or that require a PHP back-end.
You can either go to the main documentation website and download from there: https://activecss.org/docs-download.html
Alternatively if you click on the "Releases" link to the right of the repo, you can find the version that you need. The earliest version available is 2.5.1 "Elsa".
After downloading the zip file, place it somewhere sensible on your device. After unzipping the file, open up the index.html file in a modern browser (not IE) to browse the documentation. All the pages are fully bookmarkable.