If you wander a bit through the Practical Typography site, you'll realize that its whole shtick is being minimal while not looking like it was made in '94. Even the ‘Introduction’ chapter says pretty much this.
The somewhat-recent slew of ‘This is a website’ pages don't come close to PT in terms of just being nice to look at.
They're plain black but internal links are indicated with small caps and external links with Butterick's odd red degree symbol°. It seems to me to sufficiently distinguish them.
It's enough to distinguish them once you've figured it out, but it's not clear that they're links just from looking at them. I had to hover over them to be sure.
Honestly, I hadn't noticed any of the links in the text. I was merely referring to the links on the front page.
Those red circle links are particularly tricky to spot. You have no idea how many of the words prior to the circle are part of the link, so your best option is to try and click the tiny circle.
The somewhat-recent slew of ‘This is a website’ pages don't come close to PT in terms of just being nice to look at.