> parallelizing the style system in C++ was impossible within the schedule and manpower constraints Firefox was operating under

So my main issue with this claim is that we are talking about proving the negative. Not to get too pedantic, but the reasoning seems to be:

1. Mozilla tried and failed to build a parallel CSS engine in C++

2. Mozilla succeeded in building a parallel CSS engine in Rust

3. Therefore, it is impossible to implement a parallel CSS engine in C++ (under the constraints Mozilla was under).

This is simply not a valid argument. Evidence that something did not happen is not proof that it could not happen.

There are plenty of claims I would accept:

- The Mozilla team found Rust much better than C++ for solving their problem

- Many of the problems the team was running into with C++ were completely eliminated by the constraints provided by Rust

But saying this problem is impossible to solve in C++ is an extraordinary claim which needs extraordinary evidence. C++ is an extremely flexible and powerful language, to say that no one could ever design a solution to this problem using C++ requires a very limited imagination.

> there used to be at least four fairly different modern non-parallel CSS implementations out there written in C++ (Gecko, WebKit, Blink, Edge), and that one of them has disappeared (Edge), one was parallelized in Rust, and the remaining two remain in C++ and non-parallel, even though people generally agree that parallelizing them would be good and so forth.

This is a very small sample size. If I have to give a hypothesis, I can imagine that having a very large, very mature code base like Firefox would have placed a lot of design constraints on any rewrite of the CSS engine. I can imagine that working within this framework, there may have been many problems related to concurrency and data ownership which ended up eating a lot of time and energy in C++, and the team saw a huge productivity boost categorically eliminating these problems. But I simply do not believe it's the case that it's not possible to decompose the problems a CSS engine has to solve into a set of concurrent data structures and operations, which could be expressible in either Rust or C++.