There are a lot of downsides of C++ compared to more modern languages that make it not a great choice if you’re starting a web browser from scratch
Complexity of the language leading to increased bugs and slower development
Manual memory management is error-prone and leads to issues like memory leaks or segmentation faults. Modern browsers need to handle large amounts of dynamic content, making memory management complicated
C++ lacks some of the built-in safety features of more modern languages, which has led to the majority of security vulnerabilities found in major browsers. It’s so bad that Mozilla invented an entirely new programming language just to deal with this
Compared to higher-level languages, C++ can be slower to develop in, which may impact the ability to quickly implement new web standards or features unless you have a massive team
While C++ is cross-platform, ensuring consistent behavior across different operating systems can be more challenging than with some other languages.
Newer languages often provide built-in support for concurrent programming, garbage collection, and other features useful for browser development, which C++ lacks.
So tl;dr: a browser but in C++ will take much longer to develop, have fewer features, more bugs, less concurrency and and more security vulnerabilities