I distinctly remember how the web dev world (myself included) groaned at the prospect of supporting another rendering engine. Gecko was clearly the best browser at the time, and the choice of KHTML seemed bizarre, to say the least. Even the intention of Apple building it's own web browser seemed weird, Apple had failed at a previous homebrewed browser (Cyberdog), why not just build on top of what Mozilla had already…
OpenDoc, on which it was built, failed, but even that, I would not call a complete failure. It may have just been bad timing (of the Newton vs iPhone kind). If you are looking for a large hobby project: I would love to see something OpenDoc-like today running in a browser. Require viewers to be freely downloadable from e.g. GitHub, but allow developers to sell their editors, and see where it gets.