I am surprised that no one has ever mentioned this in this project before.
The world is gradually switching to HTTP3 while HTTP2 is now dominating the web. Given the performance and the security and privacy features from HTTP3 protocol, it's becoming more and more important to have HTTP3 supported. Chrome, Firefox and Safari do not have full support of HTTP3 and I think that Ladybird supports HTTP3 will form a great distinction from those. This browser is a new project in development, which means that it won't be as challenging as the other established projects to change some underlying architecture in the code without so many technical debts as the barrier.

According to the README file:

LibHTTP: HTTP/1.1 client

It seems that the HTTP library lacks of the HTTP2 and HTTP3 support. According to SerenityOS/serenity#24672, Ladybrid can use any other third party library like https://github.com/cloudflare/quiche. Given that this is written in Rust, it's also a good example of trying to integrate Rust crate into this C++ code base as a library.