Trying this out on my site. Even though I'm not too convinced about indieweb.txt, I also added my own to go with the current suggestion.

My only change is instead of relying on custom class names, use the following selector:

a[href*="benji.dog"] { color: #ffa368 }

This is what some outgoing links look like when using their citation.css values:

• shellsharks
• Reilly Spitzfaden