They'll learn so much more that won't transfer to as many job opportunities. For ex, say more about C++ and less about cutting edge CSS (because modern browser tech is an ocean). I suppose they might luck into other adjacent or unrelated roles with the same skills.
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.
They will also know the fundamental underpinnings of the web, you know - the thing that those html devs are actually using.
That’s like saying someone who fabricates cars doesn’t have the skills to drive them. Perhaps not, but they’re very well placed to pick it up quickly. They’ve also shown they can do something far more challenging, which is actually better than hiring for narrow immediate skills.
In other words: I’d hire that candidate in a heartbeat.
Are you asserting that C++ is not a marketable skill?
If anyone reading this has this un-marketable skill, Carnegie robotics in Pittsburgh is hiring
carnegie-robotics.breezy.hr/p/2d85f5321cc7-software-engineer
CSS wasn’t around in the early web.
Building a browser in the early web was actually a very achievable goal for exactly the reasons it isn’t now. There was not JS. No CSS. No SVG. In fact very few widely supported image formats (and graphical browsers weren’t around in the earliest days of the web anyway). TLS didn’t exist. HTML only had a subset of methods. And even POST was usually just managed by CGI/BIN calling an external process, often written in C++ or Perl.
It was a simpler time.