I have a hard time imagining anything where writing a browser wouldn't be an excellent transferable foundation. Even your example: writing css will never teach you as much as writing a css engine.
Sorry if I was unclear. I mean they'll never have enough time to learn a significant amount of modern web tech because they'll be working up to it from lower layers and basics.
Browsers from scratch are multi-year projects for multiple people. Even just skinning and minor tweaks to modern browsers is a deep well for one person.
