Pei-Yuan Wei's adaptation of his extensible Viola hypertext application for HTML; also the recommended browser within CERN for much of 1992. After Tim Berners-Lee's original NeXTSTEP browser, ViolaWWW was the first publicly released graphical browser.
Wei first added a WWW "app" to Viola in mid-December 1991 (he calls it "a one nite hack"), and had shared it with Tim Berners-Lee by late January 1992. Notes included with the 2.1.0 build indicate that the first versions of Viola with WWW capability were circulated at CERN in March and April 1992, with the first public releases in May and July.
The 2.x versions are precisely dated in main.c, and the 3.x versions are dated in their README files.