But I keep wondering if they could integrate at a lower-level than the source code. Like how JVM languages integrate at the bytecode level, or LLVM languages at the LLVM level
edit: looking through the docs/examples some more, it looks like javascript interop is fairly clunky, both because it relies on string concatenation to embed fragments of javascript, and because the string concatenation syntax is not great (and the formatter makes it even worse - see the example at https://github.com/anzellai/sky/blob/main/examples/13-skysho...)
I would encourage you to at the least add multiline strings with interpolation support, and ideally add a small compiler for html literals.