Adam offers a warning before he begins:
[…] I’ve been looking at ways to let users enter a destination country. Unfortunately, native HTML form controls just aren’t good enough for this type of interaction. And so we need to build a custom autocomplete control from scratch. A word of warning though: this is one of the hardest UI components I’ve ever had to make—they’re just way harder than they look.
I also just bought Adam’s book, Form Design Patterns, and this post now makes me extra excited to read it.