Before Apple, Hyatt worked at Netscape Communications from 1997 to 2002 where he contributed to the Mozilla web browser. While at Netscape, he also created Camino (then known as Chimera) and co-created Firefox (originally called Phoenix) with Blake Ross. He is credited with the implementations of tabbed browsing for Chimera and Firefox (the initial implementation for Netscape/Mozilla browsers, on which Hyatt based his work was created by HJ van Rantwijk, as part of his MultiZilla project at multizilla.mozdev.org).
Hyatt also created and wrote the first specifications for the XBL and XUL markup languages.
Hyatt developed, but no longer maintains, the software for Shadowland Six in his spare time; a forum and discussion server for the Shadowrun community. He also co-wrote published material for Shadowrun as a freelance writer, including the books Renraku Arcology: Shutdown and Brainscan.
He is also an active member of the W3C's CSS Working Group. He was an editor of the HTML5 draft specification up through March 2010, at which time he resigned to concentrate on other endeavors.