To get a short and sleek subdomain for your own GitHub Pages site follow these 4 Steps:
Step 1
If you haven't already, now it's time to log in to your GitHub account and set up your GitHub Pages site following the instructions here. To get a head start you can simply use the generator with one of the provided themes and add some reasonable content to your new page.
Step 2
Now determine your js.org subdomain: either choose your username or the name of your repo according to the existing GitHub Pages URL (for http://foo.github.io/bar, either foo.js.org or bar.js.org would be possible). More details in the wiki.
Step 3
Add a file named CNAME to your repo (in the gh-pages branch for project pages) with a single line matching the domain you have chosen (e.g. foo.js.org). If you prefer a webinterface form, have a look at GitHub Pages Help.
Step 4
To finish the procedure, make a pull request in this GitHub repository that adds your subdomain to the list of existing JS.ORG domains. Your new URL should go live within 24 hours (keep an eye on your pull request in case of a naming conflict).
Thanks
... to Cloudflare for their superb DNS service that makes this possible. While JS.ORG is using their free plan - to a shameless extent - they helped us more than once with some flexible solutions and extended quotas. Many thanks!