XMLHttpRequest XHR
Original browser API for AJAX. Microsoft, Outlook Web Access, 1999. Standardized by W3C ~2006. Drove the entire AJAX revolution (Google Maps, Gmail) → today's web.
Callback-based (onload, onerror, onprogress). Clunky API that inspired Fetch (Promise-based). Supports progress events (Fetch lacks them). Still used in: legacy code, edge cases where progress is needed, older libraries (jQuery $.ajax builds on XHR). Microsoft created the ActiveX version in 1999 for Outlook Web Access → copied by Mozilla, then everyone. The term "AJAX" was coined in 2005 by Jesse James Garrett.