Synopsis

IRIs are essentially the Unicode superset of (ASCII) URIs, combined with an ASCII embedding via UTF-8.

• RFC 3987
• Internationalized Resource Identifiers (IRIs) (W3C)

The least you should know:

• RFC 3987 §3 (IRI/URI conversion)
• HTML4 B.2.1: Non-ASCII characters in URI attribute values

Browser support

IRIs and IDNs: Testing, Implementations, and Specification Evolvement (2007)

Controlling options:

• Firefox: Bug 124042 (2002), network.standard-url.{encode-utf8,escape-utf8}
• Internet Explorer: Tools → Internet Options → Advanced → Browsing → Always send URLs as UTF-8
• Opera: Tools → Preferences → Advanced → Network → Encode international Web addresses with UTF-8

Subtleties

• Normalization, comparison, bidirectional text
• Internationalized Domain Names (RFC 3490)

Implementation

See #2409.