[jolmg]

I don't think HN shows third-level domains, so the point is moot. There may be exceptions for web services that lend out subdomains like Github[1], but doc.gwern.net would probably still show as gwern.net[2]. If you're willing to see the URL in the browser statusbar or addressbar, then the URL path makes very clear that the actual source is arxiv.org.

[1] Example: gliimly.github.io -> gliimly.github.io https://news.ycombinator.com/item?id=42148808

[2] Example: www.researchgate.net -> researchgate.net https://news.ycombinator.com/item?id=42181345

[littlestymaar]

You're right, I didn't realize that the third-level domains that show up may be due to some kind of whitelisting.

The [2] was not a convincing example because www sound something that'd get special treatment, but then I found this one:

tech.marksblogg.com -> marksblogg.com (https://news.ycombinator.com/item?id=42182519)

which proves you right. TIL.

[jolmg]

It's not even really about domains. It's about displaying the "author"/"source" of the content in the link:

https://news.ycombinator.com/item?id=23537725

> https://twitter.com/Foone/status/1011692979877105664

turns to

> (twitter.com/foone)