At a minimum:
1. You've got an incredibly clearly defined problem at the high level.
2. Extremely thorough tests for every part that build up in complexity.
3. Libraries, APIs, and tooling that are all compatible with one another because all of these technologies are built to work together already.
4. It's inherently a soft problem, you can make partial progress on it.
5. There's a reference implementation you can compare against.
6. You've got extremely detailed documentation and design docs.
7. It's a problem that inherently decomposes into separate components in a clear way.
8. The models are already trained not just on examples for every module, but on example browsers as a whole.
9. The done condition for this isn't a working browser, it's displaying something.
This isn't a realistic setup for anything that 99.99% of people work on. It's not even a realistic setup for what actual developers of browsers do who must implement new or fuzzy things that aren't in the specs.
Note 9. That's critical. Getting to the point where you can show simple pages is one thing. Getting to the point where you have a working production browser engine, that's not just 80% more work, it's probably considerably more than 100x more work.
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).