diff --git a/lld/test/wasm/driver.ll b/lld/test/wasm/driver.ll
--- a/lld/test/wasm/driver.ll
+++ b/lld/test/wasm/driver.ll
@@ -2,7 +2,7 @@
 
 target triple = "wasm32-unknown-unknown"
 
-define hidden void @entry() local_unnamed_addr #0 {
+define hidden void @_start() local_unnamed_addr #0 {
 entry:
   ret void
 }
@@ -24,3 +24,18 @@
 ; RUN: not wasm-ld --relocatable --shared-memory %t.o 2>&1 \
 ; RUN:   | FileCheck -check-prefix=SHARED-MEM %s
 ; SHARED-MEM: error: -r and --shared-memory may not be used together
+
+; RUN: wasm-ld %t.o -z foo -o /dev/null 2>&1 | FileCheck -check-prefix=ERR10 %s
+; RUN: wasm-ld %t.o -z foo -o /dev/null --version 2>&1 | FileCheck -check-prefix=ERR10 %s
+; ERR10: warning: unknown -z value: foo
+
+;; Check we report "unknown -z value" error even with -v.
+; RUN: wasm-ld %t.o -z foo -o /dev/null -v 2>&1 | FileCheck -check-prefix=ERR10 %s
+
+;; Note: in GNU ld, --fatal-warning still leads to a warning.
+; RUN: not wasm-ld %t.o -z foo --fatal-warnings 2>&1 | FileCheck --check-prefix=ERR10-FATAL %s
+; ERR10-FATAL: error: unknown -z value: foo
+
+;; stack-size without an = is also an error
+; RUN: not wasm-ld %t.o -z stack-size 2>&1 | FileCheck -check-prefix=ERR11 %s
+; ERR11: unknown -z value: stack-size
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -357,8 +357,6 @@
   config->exportAll = args.hasArg(OPT_export_all);
   config->exportTable = args.hasArg(OPT_export_table);
   config->growableTable = args.hasArg(OPT_growable_table);
-  errorHandler().fatalWarnings =
-      args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   config->importMemory = args.hasArg(OPT_import_memory);
   config->sharedMemory = args.hasArg(OPT_shared_memory);
   config->importTable = args.hasArg(OPT_import_table);
@@ -808,10 +806,28 @@
   });
 }
 
+static bool isKnownZFlag(StringRef s) {
+  // For now, we only support a very limited set of -z flags
+  return s.startswith("stack-size=");
+}
+
+// Report a warning for an unknown -z option.
+static void checkZOptions(opt::InputArgList &args) {
+  for (auto *arg : args.filtered(OPT_z))
+    if (!isKnownZFlag(arg->getValue()))
+      warn("unknown -z value: " + StringRef(arg->getValue()));
+}
+
 void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   WasmOptTable parser;
   opt::InputArgList args = parser.parse(argsArr.slice(1));
 
+  // Interpret the flags early because error()/warn() depend on them.
+  errorHandler().errorLimit = args::getInteger(args, OPT_error_limit, 20);
+  errorHandler().fatalWarnings =
+      args.hasFlag(OPT_fatal_warnings, OPT_no_fatal_warnings, false);
+  checkZOptions(args);
+
   // Handle --help
   if (args.hasArg(OPT_help)) {
     parser.printHelp(lld::outs(),
@@ -848,8 +864,6 @@
   cl::ResetAllOptionOccurrences();
   cl::ParseCommandLineOptions(v.size(), v.data());
 
-  errorHandler().errorLimit = args::getInteger(args, OPT_error_limit, 20);
-
   readConfigs(args);
 
   createFiles(args);