You don't need to fork a project. To get the repository, you can just clone it.
Go to the project page on GitHub and use the project URL to clone it:
(clicking the copy to clipboard button, well, copies the URL to the clipboard)
Open git bash and navigate to your desired folder. Then run:
git clone <your clone URL>
which in your case it would be:
git clone https://github.com/LeaVerou/prefixfree.git
Whenever new updates arrive, you can just update your repository using:
git pull origin
If you don't want the entire project history and all you need is the latest snapshot of the source files, you can click the 'Download ZIP' button to download the source files as a zip archive.
You don't even need to have an account on GitHub to perform these tasks.
The "Fork me on GitHub" ribbon is only a sign that tells the world he is using GitHub for collaborations on the project. It just directs you to the project's page.