Mozilla already tried re-writing the CSS engine in C++ before. They failed. C++ simply wasn't usable for writing a parallel CSS engine.
"This top-down structure is ripe for parallelism; however, since styling is a complex process, it’s hard to get right. Mozilla made two previous attempts to parallelize its style system in C++, and both of them failed. But Rust’s fearless concurrency has made parallelism practical!"
"This top-down structure is ripe for parallelism; however, since styling is a complex process, it’s hard to get right. Mozilla made two previous attempts to parallelize its style system in C++, and both of them failed. But Rust’s fearless concurrency has made parallelism practical!"
https://blog.rust-lang.org/2017/11/14/Fearless-Concurrency-I...