Whether it is the best case scenario in terms of benchmark, I am not so sure.
The Web is indeed standardized and there are many open-source implementations out there. But how to implement the Web in a novel way by definition means you are trying to solve some perceived problem with existing implementations.
So I would rephrase your statement as such: rewriting an existing engine in another language without any novelty might be the best case scenario for autonomous coding agents.
As an example of approaching the problem in a novel way: the Fastrender code seems obsessed with metering of resources. Implementing the Web with that constraint in mind would be an interesting problem and not obvious at all. That's not what the project is doing so far by the way, since the code is quite frankly a bunch of spaghetti that does not follow Web standards at all(in a way that is unrelated to the metering story, so the divergence from specs is not novel, it's just wrong).