Faust is a functional programming language designed specifically for digital signal processing. It is an older language developed in academia and supports a functional design and support for a huge variety of platforms from microcontrollers to the web browser via WebAudio + WebAssembly.
Faust is well-known in the digital signal processing world, and many open-source programs exist that can be dropped directly into web-synth with zero porting required. For example, I've used this model of the Buchla 259 wavefolder in web-synth directly and it works like a charm with no modifications necessary.
It is licensed GPLv2, but iirc there are some exemptions for certain things. In particular, I know that the Faust standard library is licensed LGPL but has a condition such that it can be used outside o LGPL context:
EXCEPTION TO THE LGPL LICENSE : As a special exception, you may create a
larger FAUST program which directly or indirectly imports this library
file and still distribute the compiled code generated by the FAUST
compiler, or a modified version of this compiled code, under your own
copyright and license.
Faust is integrated directly into web synth via the [code-editor].