At least for servo, it started as a research project. I imagine a lot of time was spent trying out ideas for WebRender or parallel CSS (two parts that made it i

At least for servo, it started as a research project. I imagine a lot of time was spent trying out ideas for WebRender or parallel CSS (two parts that made it into Firefox). I also assume starting a browser i easier than finishing a browser but I can't speak to where either is at on the difficut-to-finish spectrum.