U_FFFD
A strategy which replaces an invalid character sequence with a 3-byte sequence of 0xef, 0xbf, 0xbd (i.e. character �) during text to UTF-8 byte transformations, and will replace partial surrogate code points with 3 replacement � characters during UTF-8 byte to text transformations.
This strategy is reflective of how Kotlin Js/WasmJs/Wasi/Native encodes/decodes UTF-8.