Author here: my website isn't about Javascript in the first place, it's about compression.
I could have done something similar with PNGs, PDFs or ZIPs (same DEFLATE compression algorithm) but Javascript is much more widespread.
<script>window.jQuery || document.write('<script src="local_server_path/jquery.min.js"></script>')</script>
In the browser I always end up with a ') at the top of all my pages. I'm assuming this is because the script tag actually closes inside of the document.write instead of the actual end. Am I the only one with this issue? Not sure what went wrong here.