I don't understand the use of JS benchmarks to compare Servo to Ladybird.
Ladybird indeed has its own JS engine, and it's very young and still interpreted.
On the other hand, Servo uses SpiderMonkey (i.e. shipping C++ code from Mozilla) as its JS engine.
So a Servo vs Ladybird comparison using JS benchmarks is really a comparison of Mozilla's shipping JS engine to Ladybird's from-scratch JS engine. And they're both written in C++.
Ladybird indeed has its own JS engine, and it's very young and still interpreted.
On the other hand, Servo uses SpiderMonkey (i.e. shipping C++ code from Mozilla) as its JS engine.
So a Servo vs Ladybird comparison using JS benchmarks is really a comparison of Mozilla's shipping JS engine to Ladybird's from-scratch JS engine. And they're both written in C++.