diff --git a/libcxx/include/__config_site b/libcxx/include/__config_site
new file mode 100644
index 0000000000000..c1c848c80aba8
--- /dev/null
+++ b/libcxx/include/__config_site
@@ -0,0 +1,10 @@
+// Set the LIBCPP ABI version 2 under emscripten so that we get nicely aligned string
+// data and other nice fixes.
+#define _LIBCPP_ABI_VERSION 2
+#define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
+#define _LIBCPP_HAS_MUSL_LIBC
+#define _LIBCPP_ABI_NAMESPACE __2
+// Emscripten doesn't use PSTL at the moment.
+#define _LIBCPP_PSTL_CPU_BACKEND_SERIAL
+// __libcpp_verbose_abort creases code size too much
+#define _LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT