Interesting to see Viola mentioned so prominantly here, and to see its impact on Andreesen, Mosaic, and Netscape. Whilst I'd never used it myself, I ran across it a year or so back when looking at how the n=modrn browser mechanics were estabished: navigation, history, bookmarks, scropting. Viola extended many of these concepts, and explored ground that's since been lost; Applets, scripting (Tcl), stylesheets, frames, forms, equations editor, columnar layouts, sidebar as integral element, and more.

Its descriptive pages are well worth revisiting.

http://www.viola.org