All those other engines are cross-platform, whereas LibJS is UNIX only and actually very dependent on SerenityOS libraries (so not really a portable UNIX library), but that's apparently the intended design.
You can totally build and use V8 independently (many projects, including node.js, do it) and they have documentation specifically targeted at embedders.
JSC is not as nice but can still be built and embedded as a stand-alone library.
Yes but in doing so you will also build the supporting C++ library. I imagine it would be relatively easy to do the same for the serenity javascript engine / browser. The serenity standard library https://github.com/SerenityOS/serenity/blob/master/Meta/Lago... has support for other platforms.
It would be nice if LibJS would become an independent library, but last time I checked it had way too many dependencies on other core / helper libraries in SerenityOS for it to be ripped out; not sure why it would need a separate website unless making it independent is in the plans.
Being closely integrated with the rest of the system & libraries is a feature, not a bug - no plans to change anything about it :^)
That being said, it's definitely possible to use it outside of SerenityOS as a regular library, just need a checkout of the serenity source for that.
See https://github.com/linusg/libjs-test262 for an example, specifically the CMake files and `src/main.cpp`.
The separate website is just to have a nice & short link to the test262 graph and whatever else we'll put there eventually; and also because it's built/deployed in a slightly different way than the main serenityos.org website.
OP links to a not quite informative web site.