To get a short and sleek subdomain for your own GitHub Pages site from JS.ORG follow these 4 steps:

If you haven't already, log in to your GitHub account and set up your GitHub Pages site following their instructions. Make sure to add some reasonable content to your new page.

Now determine your JS.ORG subdomain: either choose your username or the name of your repository according to the existing GitHub Pages URL (for http://foo.github.io/bar, either foo.js.org or bar.js.org would be possible). We have more guidance on picking an appropriate subdomain in our wiki: https://github.com/js-org/js.org/wiki/Subdomain-Determination.

If you're publishing from a branch:

Add a file named CNAME to your repo (in the gh-pages branch for project pages, or the branch that you've set as your GitHub Pages source) with a single line matching the domain you have chosen (e.g. foo.js.org). You can also use the UI in the repository settings as discussed below, if desired.

If you're publishing using a workflow:

A CNAME file will not be processed when publishing a site via a workflow, so you will need to use the UI in the repository settings to add the custom domain. Head into the repository settings and under the Pages tab add your JS.ORG subdomain as a custom domain.

To finish the procedure, make a pull request in this GitHub repository that adds your subdomain to the subdomains 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 or if there are requested changes).

If you'd like to use JS.ORG for a website hosted elsewhere, you can do that too!

If you haven't already, get your site set up with whatever hosting provider you desire. Note that to use JS.ORG, your hosting provider will need to support adding custom domains via a CNAME DNS record.

Now determine your JS.ORG subdomain: either choose your username or the name of your repository according to the existing GitHub Pages URL (for http://foo.github.io/bar, either foo.js.org or bar.js.org would be possible). We have more guidance on picking an appropriate subdomain in our wiki: https://github.com/js-org/js.org/wiki/Subdomain-Determination.

Follow your hosting provider's instructions for adding a custom domain to your site, if configuration is needed. Our wiki contains a list of hosting providers we've seen folks successfully use in the past, including some notes on correctly configuring some of them: https://github.com/js-org/js.org/wiki/3rd-party-hosts.

As above with adding a subdomain to a GitHub Pages site, the final step is to make a pull request in this GitHub repository that adds your subdomain to the subdomains list of existing JS.ORG domains.

Thanks to Cloudflare for their awesome DNS service that makes this service 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!