Only to bootstrap the language, it's certainly not the plan to require that forever.

That's kind of interesting that it currently transpiles to C++. I have a suspicion though that using C++ as an intermediate language will probably often result in lower performance, higher memory usage, make debugging more complicated, etc... than if it was compiled directly to a lower level language like assembly or LLVM IR. If I'm right about that, those wouldn't be very good characteristics for a programming language that you intend to make a complex web browser in.

In zamadatix's reply to your message he said that C++ is only being used to bootstrap the language and that it's certainly not the plan to require C++ forever. I imagine if Jakt thrives, he will be right and Jakt will probably start compiling into a lower level language instead. If they do target a lower level language though, I imagine this will result in Jakt being less portable than C++ which has been around much longer and is more established.