You are currently looking at the < v8.2.0 docs (Reason v3.6 syntax edition). You can find the latest manual page here.
(These docs cover all versions between v3 to v8 and are equivalent to the old BuckleScript docs before the rebrand)
Browser Support & Polyfills
ReScript compiles to JavaScript ES5, with the exception of optionally allowing to compile to ES6's module import & export.
For old browsers, you also need to polyfill TypedArray. The following standard library functions require it:
Int64.float_of_bits
Int64.bits_of_float
Int32.float_of_bits
Int32.bits_of_float
If you don't use these functions, you're fine. Otherwise, it'll be a runtime failure.