diff --git a/libcxx/docs/Status/Cxx2bIssues.csv b/libcxx/docs/Status/Cxx2bIssues.csv
--- a/libcxx/docs/Status/Cxx2bIssues.csv
+++ b/libcxx/docs/Status/Cxx2bIssues.csv
@@ -183,7 +183,7 @@
 "`3713 <https://wg21.link/LWG3713>`__","Sorted with respect to comparator (only)","July 2022","",""
 "`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","",""
 "`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","",""
-"`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","","","|format|"
+"`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","|Complete|","16.0","|format|"
 "`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
 "","","","",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","Not voted in","|Complete|","14.0",""
diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -534,10 +534,7 @@
     if (!__width_as_arg_)
       return __width_;
 
-    int32_t __result = __format_spec::__substitute_arg_id(__ctx.arg(__width_));
-    if (__result == 0)
-      __throw_format_error("A format-spec width field replacement should have a positive value");
-    return __result;
+    return __format_spec::__substitute_arg_id(__ctx.arg(__width_));
   }
 
   _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/test/std/utilities/format/format.functions/format_tests.h b/libcxx/test/std/utilities/format/format.functions/format_tests.h
--- a/libcxx/test/std/utilities/format/format.functions/format_tests.h
+++ b/libcxx/test/std/utilities/format/format.functions/format_tests.h
@@ -241,6 +241,9 @@
   check_exception("A format-spec width field shouldn't have a leading zero", SV("hello {:0}"), world);
 
   // *** width ***
+  // Width 0 allowed, but not useful for string arguments.
+  check.template operator()<"hello {:{}}">(SV("hello world"), world, 0);
+
 #ifdef _LIBCPP_VERSION
   // This limit isn't specified in the Standard.
   static_assert(std::__format::__number_max == 2'147'483'647, "Update the assert and the test.");
@@ -249,7 +252,6 @@
   check_exception("The numeric value of the format-spec is too large", SV("{:10000000000}"), world);
 #endif
 
-  check_exception("A format-spec width field replacement should have a positive value", SV("hello {:{}}"), world, 0);
   check_exception("A format-spec arg-id replacement shouldn't have a negative value", SV("hello {:{}}"), world, -1);
   check_exception("A format-spec arg-id replacement exceeds the maximum supported value", SV("hello {:{}}"), world,
                   unsigned(-1));