diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
index 3c4690a398..87a037c26a 100644
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -12,14 +12,16 @@ config("find_bad_constructs") {
 # Enables some extra Clang-specific warnings. Some third-party code won't
 # compile with these so may want to remove this config.
 config("extra_warnings") {
-  cflags = [
-    # Warns when a const char[] is converted to bool.
-    "-Wstring-conversion",
+  if (!is_win) {
+    cflags = [
+      # Warns when a const char[] is converted to bool.
+      "-Wstring-conversion",
 
-    # Warns when a source file doesn't have a newline at end-of-file.
-    # This is to match Fuchsia, which enables this warning.
-    "-Wnewline-eof",
-  ]
+      # Warns when a source file doesn't have a newline at end-of-file.
+      # This is to match Fuchsia, which enables this warning.
+      "-Wnewline-eof",
+    ]
+  }
 
   defines = [ "_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS" ]
 }
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 4f3544f93e..286fa2b247 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -78,6 +78,8 @@ config("compiler") {
       "/GS",  # Enable buffer security checking.
       "/FS",  # Preserve previous PDB behavior.
     ]
+
+    ldflags += [ "/ignore:4217" ]
   } else {
     # Common GCC compiler flags setup.
     # --------------------------------
@@ -324,6 +326,7 @@ config("compiler") {
       "STRICT=1",
       "-s",
       "WARN_UNALIGNED=1",
+
       # Reduces global namespace pollution.
       "-s",
       "MODULARIZE=1",
@@ -535,6 +538,8 @@ config("runtime_library") {
       "_CRT_SECURE_NO_DEPRECATE",
       "_HAS_EXCEPTIONS=0",
       "_SCL_SECURE_NO_DEPRECATE",
+      "_CRT_NONSTDC_NO_WARNINGS",
+      "_CRT_NONSTDC_NO_DEPRECATE",
     ]
   }
 
@@ -642,14 +647,35 @@ if (is_win) {
     "/wd4311",  # Pointer truncation from PVOID to DWORD.
     "/wd4477",  # Format string requires wchar_t*
   ]
-  import("//build/toolchain/goma.gni")
-  if (is_clang && use_goma) {
-    # goma compilation doesn't preserve file case, so don't warn about
-    # case-mismatches.
-    default_warning_flags += [
+
+  if (is_clang) {
+    # These warning flags seem to be Clang specific in addition to those
+    # specified for MSVC above.
+    extra_warning_flags = [
+      # Warnings as errors.
+      "-Werror",
+
+      # Some third party dependencies like ICU add flags that are not supported
+      # by clang-cl yet.
+      "-Wno-unknown-argument",
+
       "-Wno-nonportable-include-path",
       "-Wno-nonportable-system-include-path",
+      "-Wno-implicit-int-float-conversion",
+      "-Wno-double-promotion",
+      "-Wno-ignored-pragma-optimize",
+      "-Wno-microsoft-unqualified-friend",
+      "-Wno-microsoft-cast",
+      "-Wno-tautological-constant-out-of-range-compare",
+      "-Wno-string-compare",
+      "-Wno-undefined-var-template",
+
+      # TODO: There only seems to one legitimate case of this happening in Dart.
+      # Patch that callsite or move the flag to Dart GN rules.
+      "-Wno-deprecated-declarations",
     ]
+    default_warning_flags += extra_warning_flags
+    default_warning_flags_cc += extra_warning_flags
   }
 } else {
   # Common GCC warning setup.
@@ -667,6 +693,7 @@ if (is_win) {
     default_warning_flags += [
       # zlib needs this
       "-Wno-shift-negative-value",
+
       # freetype2 needs these two
       "-Wno-unused-function",
       "-Wno-unused-variable",
@@ -919,8 +946,11 @@ config("no_optimize") {
     }
     ldflags = common_optimize_on_ldflags
   } else if (is_wasm) {
-    cflags = [ "-O0", "-g4" ]
-  } else  {
+    cflags = [
+      "-O0",
+      "-g4",
+    ]
+  } else {
     cflags = [ "-O0" ]
   }
 }
diff --git a/build/config/gcc/BUILD.gn b/build/config/gcc/BUILD.gn
index 175db0c756..9150ef1ad6 100644
--- a/build/config/gcc/BUILD.gn
+++ b/build/config/gcc/BUILD.gn
@@ -19,7 +19,10 @@ config("symbol_visibility_hidden") {
   # config since that can almost always be applied.
   if (!enable_profiling) {
     defines = [ "_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS" ]
-    cflags = [ "-fvisibility=hidden" ]
+
+    if (is_clang && !is_win) {
+      cflags = [ "-fvisibility=hidden" ]
+    }
   }
 }