diff --git a/build/BUILD.gn b/build/BUILD.gn
index 58f5f20fb4b4..663413278785 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -40,6 +40,7 @@ buildflag_header("chromecast_buildflags") {
   header = "chromecast_buildflags.h"
 
   flags = [
+    "IS_CHROMECAST=$is_chromecast",
     "IS_CASTOS=$is_castos",
     "IS_CAST_ANDROID=$is_cast_android",
     "ENABLE_CAST_RECEIVER=$enable_cast_receiver",
diff --git a/build/android/docs/coverage.md b/build/android/docs/coverage.md
index c7f3c1ffe6e0..2f362f687acf 100644
--- a/build/android/docs/coverage.md
+++ b/build/android/docs/coverage.md
@@ -29,6 +29,10 @@ class files and runtime **.exec** files. Then we need to process them using the
 3. The coverage results of JUnit and instrumentation tests will be merged
    automatically if they are in the same directory.
 
+4. If generating coverage and there are duplicate class files, as can happen
+   when generating coverage for downstream targets, use the
+   --include-substr-filter option to choose jars in the desired directory.
+
 ## How to generate coverage report
 
 1. Now we have generated .exec files already. We can create a JaCoCo HTML/XML/CSV
diff --git a/build/android/pylib/gtest/gtest_test_instance_test.py b/build/android/pylib/gtest/gtest_test_instance_test.py
index c714ba0cfc9e..6502f5f0e04c 100755
--- a/build/android/pylib/gtest/gtest_test_instance_test.py
+++ b/build/android/pylib/gtest/gtest_test_instance_test.py
@@ -312,6 +312,37 @@ def testParseGTestJSON_skippedTest_example(self):
                      actual[0].GetName())
     self.assertEqual(base_test_result.ResultType.SKIP, actual[0].GetType())
 
+  def testParseGTestJSON_skippedTest_example(self):
+    raw_json = """
+      {
+        "tests": {
+          "mojom_tests": {
+            "parse": {
+              "ast_unittest": {
+                "ASTTest": {
+                  "testNodeBase": {
+                    "expected": "SKIP",
+                    "actual": "SKIP",
+                  }
+                }
+              }
+            }
+          }
+        },
+        "interrupted": false,
+        "path_delimiter": ".",
+        "version": 3,
+        "seconds_since_epoch": 1406662283.764424,
+        "num_failures_by_type": {
+          "SKIP": 1
+        },
+      }"""
+    actual = gtest_test_instance.ParseGTestJSON(raw_json)
+    self.assertEquals(1, len(actual))
+    self.assertEquals('mojom_tests.parse.ast_unittest.ASTTest.testNodeBase',
+                      actual[0].GetName())
+    self.assertEquals(base_test_result.ResultType.SKIP, actual[0].GetType())
+
   def testTestNameWithoutDisabledPrefix_disabled(self):
     test_name_list = [
       'A.DISABLED_B',
diff --git a/build/android/pylib/local/device/local_device_test_run.py b/build/android/pylib/local/device/local_device_test_run.py
index 0e0b93033400..e3862d14b497 100644
--- a/build/android/pylib/local/device/local_device_test_run.py
+++ b/build/android/pylib/local/device/local_device_test_run.py
@@ -99,6 +99,13 @@ def run_tests_on_device(dev, tests, results):
           # of bad device detection.
           consecutive_device_errors = 0
 
+          # TODO(crbug.com/1181389): Remove this workaround once the deadlocks
+          # in ArCore are resolved
+          def GetResultTypeForTest(t):
+            if 'WebXrAr' in self._GetUniqueTestName(t):
+              return base_test_result.ResultType.PASS
+            return base_test_result.ResultType.TIMEOUT
+
           if isinstance(test, list):
             results.AddResults(
                 base_test_result.BaseTestResult(
diff --git a/build/android/pylib/results/json_results_test.py b/build/android/pylib/results/json_results_test.py
index 6cf6487e54f8..8b2cb3623abc 100755
--- a/build/android/pylib/results/json_results_test.py
+++ b/build/android/pylib/results/json_results_test.py
@@ -270,6 +270,34 @@ def testGenerateJsonTestResultFormatDict_skippedResult(self):
     self.assertIn('SKIP', results_dict['num_failures_by_type'])
     self.assertEqual(1, results_dict['num_failures_by_type']['SKIP'])
 
+  def testGenerateJsonTestResultFormatDict_skippedResult(self):
+    result = base_test_result.BaseTestResult('test.package.TestName',
+                                             base_test_result.ResultType.SKIP)
+
+    all_results = base_test_result.TestRunResults()
+    all_results.AddResult(result)
+
+    results_dict = json_results.GenerateJsonTestResultFormatDict([all_results],
+                                                                 False)
+    self.assertEquals(1, len(results_dict['tests']))
+    self.assertEquals(1, len(results_dict['tests']['test']))
+    self.assertEquals(1, len(results_dict['tests']['test']['package']))
+    self.assertEquals(
+        'PASS',
+        results_dict['tests']['test']['package']['TestName']['expected'])
+    self.assertEquals(
+        'FAIL', results_dict['tests']['test']['package']['TestName']['actual'])
+    self.assertEquals(
+        True,
+        results_dict['tests']['test']['package']['TestName']['is_unexpected'])
+
+    self.assertTrue('FAIL' not in results_dict['num_failures_by_type']
+                    or results_dict['num_failures_by_type']['FAIL'] == 0)
+    self.assertTrue('PASS' not in results_dict['num_failures_by_type']
+                    or results_dict['num_failures_by_type']['PASS'] == 0)
+    self.assertIn('SKIP', results_dict['num_failures_by_type'])
+    self.assertEquals(1, results_dict['num_failures_by_type']['SKIP'])
+
   def testGenerateJsonTestResultFormatDict_failedResultWithRetry(self):
     result_1 = base_test_result.BaseTestResult('test.package.TestName',
                                                base_test_result.ResultType.FAIL)
diff --git a/build/build_config.h b/build/build_config.h
index 6db5d9bca059..798b49467a07 100644
--- a/build/build_config.h
+++ b/build/build_config.h
@@ -55,7 +55,9 @@
 #include "build/buildflag.h"  // IWYU pragma: export
 
 // A set of macros to use for platform detection.
-#if defined(__native_client__)
+#if defined(STARBOARD)
+// noop
+#elif defined(__native_client__)
 // __native_client__ must be first, so that other OS_ defines are not set.
 #define OS_NACL 1
 #elif defined(ANDROID)
@@ -268,7 +270,33 @@
 //   http://msdn.microsoft.com/en-us/library/b0084kay.aspx
 //   http://www.agner.org/optimize/calling_conventions.pdf
 //   or with gcc, run: "echo | gcc -E -dM -"
-#if defined(_M_X64) || defined(__x86_64__)
+#if defined(STARBOARD)
+#include "starboard/configuration.h"
+#if SB_IS(32_BIT)
+#define ARCH_CPU_32_BITS 1
+#elif SB_IS(64_BIT)
+#define ARCH_CPU_64_BITS 1
+#endif  // SB_IS(32_BIT)
+#if SB_IS(BIG_ENDIAN)
+#define ARCH_CPU_BIG_ENDIAN 1
+#else   // SB_IS(BIG_ENDIAN)
+#define ARCH_CPU_LITTLE_ENDIAN 1
+#endif  // SB_IS(BIG_ENDIAN)
+#if SB_IS(ARCH_X86)
+#define ARCH_CPU_X86_FAMILY 1
+#define ARCH_CPU_X86 1
+#elif SB_IS(ARCH_X64)
+#define ARCH_CPU_X86_FAMILY 1
+#define ARCH_CPU_X86_64 1
+#elif SB_IS(ARCH_ARM) || SB_IS(ARCH_ARM64)
+#define ARCH_CPU_ARM_FAMILY 1
+#if SB_IS(BIG_ENDIAN)
+#define ARCH_CPU_ARM 1
+#else   // SB_IS(BIG_ENDIAN)
+#define ARCH_CPU_ARMEL 1
+#endif  // SB_IS(BIG_ENDIAN)
+#endif
+#elif defined(_M_X64) || defined(__x86_64__)
 #define ARCH_CPU_X86_FAMILY 1
 #define ARCH_CPU_X86_64 1
 #define ARCH_CPU_64_BITS 1
@@ -355,7 +383,13 @@
 #endif
 
 // Type detection for wchar_t.
-#if defined(OS_WIN)
+#if defined(STARBOARD)
+#if SB_IS(WCHAR_T_UTF16)
+#define WCHAR_T_IS_UTF16
+#elif SB_IS(WCHAR_T_UTF32)
+#define WCHAR_T_IS_UTF32
+#endif
+#elif defined(OS_WIN)
 #define WCHAR_T_IS_UTF16
 #elif defined(OS_FUCHSIA)
 #define WCHAR_T_IS_UTF32
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 749b0855ff2a..2106261c1f2c 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -245,7 +245,7 @@ group("common_deps") {
     public_deps += [ "//build/config/sanitizers:deps" ]
   }
 
-  if (use_custom_libcxx) {
+  if (!is_starboard && use_custom_libcxx) {
     public_deps += [ "//buildtools/third_party/libc++" ]
   }
 
diff --git a/build/config/android/internal_rules.gni b/build/config/android/internal_rules.gni
index 41abf539fe96..427fa0dca401 100644
--- a/build/config/android/internal_rules.gni
+++ b/build/config/android/internal_rules.gni
@@ -15,7 +15,7 @@ import("//build/toolchain/goma.gni")
 import("//build/toolchain/kythe.gni")
 import("//build/util/generate_wrapper.gni")
 import("//build_overrides/build.gni")
-if (current_toolchain == default_toolchain) {
+if (is_starboardized_toolchain || current_toolchain == default_toolchain) {
   import("//build/toolchain/concurrent_links.gni")
 }
 assert(is_android)
diff --git a/build/config/apple/sdk_info.py b/build/config/apple/sdk_info.py
index 81b06d438df1..bac73feec791 100755
--- a/build/config/apple/sdk_info.py
+++ b/build/config/apple/sdk_info.py
@@ -134,7 +134,8 @@ def main():
                       default='.',
                       help='Value of gn $root_build_dir')
   parser.add_argument('platform',
-                      choices=['iphoneos', 'iphonesimulator', 'macosx'])
+                      choices=['iphoneos', 'iphonesimulator', 'macosx',
+                               'appletvos'])
   args = parser.parse_args()
   if args.developer_dir:
     os.environ['DEVELOPER_DIR'] = args.developer_dir
diff --git a/build/config/arm.gni b/build/config/arm.gni
index cc82ed5ba922..8d180ad64c4b 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -5,6 +5,10 @@
 import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/v8_target_cpu.gni")
 
+if (is_starboard) {
+  import("//starboard/sabi/sabi.gni")
+}
+
 # These are primarily relevant in current_cpu == "arm" contexts, where
 # ARM code is being compiled.  But they can also be relevant in the
 # other contexts when the code will change its behavior based on the
@@ -45,14 +49,19 @@ if (current_cpu == "arm" || v8_current_cpu == "arm") {
     arm_arch = "armv8-a+crc"
   }
 
-  if (current_os == "android" || target_os == "android") {
-    arm_float_abi = "softfp"
+  if (is_starboard) {
+    arm_float_abi = sabi_variables.floating_point_abi
+    arm_fpu = sabi_variables.floating_point_fpu
   } else {
-    declare_args() {
-      # The ARM floating point mode. This is either the string "hard", "soft",
-      # or "softfp". An empty string means to use the default one for the
-      # arm_version.
-      arm_float_abi = ""
+    if (current_os == "android" || target_os == "android") {
+      arm_float_abi = "softfp"
+    } else {
+      declare_args() {
+        # The ARM floating point mode. This is either the string "hard", "soft",
+        # or "softfp". An empty string means to use the default one for the
+        # arm_version.
+        arm_float_abi = ""
+      }
     }
   }
   assert(arm_float_abi == "" || arm_float_abi == "hard" ||
diff --git a/build/config/clang/clang.gni b/build/config/clang/clang.gni
index 1aad3d60b840..ff978bcd0dde 100644
--- a/build/config/clang/clang.gni
+++ b/build/config/clang/clang.gni
@@ -4,7 +4,9 @@
 
 import("//build/toolchain/toolchain.gni")
 
-default_clang_base_path = "//third_party/llvm-build/Release+Asserts"
+if (!use_cobalt_customizations) {
+  default_clang_base_path = "//third_party/llvm-build/Release+Asserts"
+}
 
 declare_args() {
   # Indicates if the build should use the Chrome-specific plugins for enforcing
@@ -18,5 +20,7 @@ declare_args() {
       build_with_chromium && !is_official_build &&
       ((is_linux && !is_castos) || (is_android && !is_cast_android))
 
-  clang_base_path = default_clang_base_path
+  if (!use_cobalt_customizations) {
+    clang_base_path = default_clang_base_path
+  }
 }
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 28c2255ba809..12742fd5c37b 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -12,7 +12,9 @@ import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/coverage/coverage.gni")
 import("//build/config/dcheck_always_on.gni")
+if (!use_cobalt_customizations) {
 import("//build/config/gclient_args.gni")
+}
 import("//build/config/host_byteorder.gni")
 import("//build/config/rust.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ -1407,7 +1409,8 @@ config("compiler_deterministic") {
 }
 
 config("clang_revision") {
-  if (is_clang && clang_base_path == default_clang_base_path) {
+  if (!use_cobalt_customizations && is_clang &&
+      clang_base_path == default_clang_base_path) {
     update_args = [
       "--print-revision",
       "--verify-version=$clang_version",
@@ -1481,7 +1484,7 @@ config("runtime_library") {
   # android:runtime_library.  This is to ensure libc++ appears before
   # libandroid_support in the -isystem include order.  Otherwise, there will be
   # build errors related to symbols declared in math.h.
-  if (use_custom_libcxx) {
+  if (!use_cobalt_customizations && use_custom_libcxx) {
     configs += [ "//build/config/c++:runtime_library" ]
   }
 
@@ -1565,7 +1568,7 @@ config("default_warnings") {
 
     # Suppress warnings about ABI changes on ARM (Clang doesn't give this
     # warning).
-    if (current_cpu == "arm" && !is_clang) {
+    if (!is_starboard && current_cpu == "arm" && !is_clang) {
       cflags += [ "-Wno-psabi" ]
     }
 
@@ -1610,7 +1613,7 @@ config("default_warnings") {
       "-Wno-unused-parameter",  # Unused function parameters.
     ]
 
-    if (!is_nacl || is_nacl_saigo) {
+    if (!is_starboard && (!is_nacl || is_nacl_saigo)) {
       cflags += [
         # An ABI compat warning we don't care about, https://crbug.com/1102157
         # TODO(thakis): Push this to the (few) targets that need it,
@@ -1628,8 +1631,19 @@ config("default_warnings") {
       # which we no longer use. Check if it makes sense to remove
       # this as well. http://crbug.com/316352
       "-Wno-unneeded-internal-declaration",
+
+      "-Wno-extra-semi",
+      "-Wno-pessimizing-move",
+      "-Wno-shadow",
     ]
 
+    if (use_cobalt_customizations) {
+      cflags += [
+        "-Wno-range-loop-bind-reference",
+        "-Wno-range-loop-construct",
+      ]
+    }
+
     if (!is_nacl || is_nacl_saigo) {
       if (is_win) {
         # TODO(thakis): https://crbug.com/617318
@@ -1637,15 +1651,17 @@ config("default_warnings") {
         cflags += [ "-Wno-nonportable-include-path" ]
       }
 
-      cflags += [
-        "-Wenum-compare-conditional",
+      if (!use_cobalt_customizations) {
+        cflags += [
+          "-Wenum-compare-conditional",
 
-        # Ignore warnings about MSVC optimization pragmas.
-        # TODO(thakis): Only for no_chromium_code? http://crbug.com/912662
-        "-Wno-ignored-pragma-optimize",
-      ]
+          # Ignore warnings about MSVC optimization pragmas.
+          # TODO(thakis): Only for no_chromium_code? http://crbug.com/912662
+          "-Wno-ignored-pragma-optimize",
+        ]
+      }
 
-      if (!is_nacl) {
+      if (!use_cobalt_customizations && !is_nacl) {
         cflags += [
           # TODO(crbug.com/1343975) Evaluate and possibly enable.
           "-Wno-deprecated-builtins",
@@ -1685,13 +1701,18 @@ config("default_warnings") {
 #   configs += [ "//build/config/compiler:prevent_unsafe_narrowing" ]
 
 config("prevent_unsafe_narrowing") {
+  cflags = []
   if (is_clang) {
-    cflags = [
-      "-Wshorten-64-to-32",
+    cflags += [
       "-Wimplicit-int-conversion",
       "-Wsign-compare",
       "-Wsign-conversion",
     ]
+    if (!is_starboard) {
+      cflags += [
+        "-Wshorten-64-to-32",
+      ]
+    }
     if (!is_nacl) {
       cflags += [
         # Avoid bugs of the form `if (size_t i = size; i >= 0; --i)` while
@@ -1709,12 +1730,24 @@ config("prevent_unsafe_narrowing") {
 
 config("chromium_code") {
   if (is_win) {
-    if (is_clang) {
-      cflags = [ "/W4" ]  # Warning level 4.
+    if (is_starboard) {
+      # The platform should set warning flags.
+      cflags = []
+    } else {
+      if (is_clang) {
+        cflags = [ "/W4" ]  # Warning level 4.
 
-      # Opt in to additional [[nodiscard]] on standard library methods.
-      defines = [ "_HAS_NODISCARD" ]
+        # Opt in to additional [[nodiscard]] on standard library methods.
+        defines = [ "_HAS_NODISCARD" ]
+      }
     }
+  } else if (is_starboard) {
+    # TODO(b/205790602): Revisit this code to be more compatible with platforms.
+    defines = [
+      "__STDC_CONSTANT_MACROS",
+      "__STDC_FORMAT_MACROS",
+    ]
+    cflags = [ "-Werror" ]
   } else {
     cflags = [ "-Wall" ]
     if (treat_warnings_as_errors) {
@@ -1773,13 +1806,38 @@ config("chromium_code") {
       # For intentional fallthrough, use [[fallthrough]].
       "-Wimplicit-fallthrough",
 
-      # Warn on unnecessary extra semicolons outside of function definitions.
-      "-Wextra-semi",
     ]
 
+    if (!is_starboard) {
+      cflags += [
+        # Warn on unnecessary extra semicolons outside of function definitions.
+        "-Wextra-semi",
+      ]
+    }
+
+    # Suppress warning in old //net.
+    if (is_starboard) {
+      cflags += [
+        "-Wno-reorder-ctor",
+        "-Wno-unused-const-variable",
+        "-Wno-unused-variable",
+        "-Wno-unused-private-field",
+        "-Wno-missing-braces",
+        "-Wno-string-concatenation",
+      ]
+    }
+
+    # Suppress warnings in old //base and //net.
+    if (is_starboard) {
+      cflags += [
+        "-Wno-sign-compare",
+        "-Wno-shorten-64-to-32",
+      ]
+    }
+
     # TODO(thakis): Enable this more often, https://crbug.com/346399
     # use_fuzzing_engine_with_lpm: https://crbug.com/1063180
-    if ((!is_nacl || is_nacl_saigo) && !use_fuzzing_engine_with_lpm) {
+    if (!is_starboard && (!is_nacl || is_nacl_saigo) && !use_fuzzing_engine_with_lpm) {
       cflags += [ "-Wunreachable-code-aggressive" ]
     }
 
@@ -1793,10 +1851,13 @@ config("chromium_code") {
     }
   }
 
-  configs = [
-    ":default_warnings",
-    ":noshadowing",
-  ]
+  configs = [ ":default_warnings" ]
+
+  if (!is_starboard) {
+    configs += [
+      ":noshadowing",
+    ]
+  }
 }
 
 config("no_chromium_code") {
@@ -1805,7 +1866,7 @@ config("no_chromium_code") {
   defines = []
 
   if (is_win) {
-    if (is_clang) {
+    if (!is_starboard && is_clang) {
       cflags += [ "/W3" ]  # Warning level 3.
     }
     cflags += [
@@ -1836,7 +1897,8 @@ config("no_chromium_code") {
       # third-party libraries.
       "-Wno-c++11-narrowing",
     ]
-    if (!is_nacl) {
+    if (!use_cobalt_customizations && !is_nacl &&
+        (current_toolchain == host_toolchain || !use_xcode_clang)) {
       cflags += [
         # Disabled for similar reasons as -Wunused-variable.
         "-Wno-unused-but-set-variable",
@@ -1985,6 +2047,50 @@ config("wexit_time_destructors") {
   }
 }
 
+# On Windows compiling on x64, VC will issue a warning when converting
+# size_t to int because it will truncate the value. Our code should not have
+# these warnings and one should use a static_cast or a checked_cast for the
+# conversion depending on the case. However, a lot of code still needs to be
+# fixed. Apply this config to such targets to disable the warning.
+#
+# Note that this can be applied regardless of platform and architecture to
+# clean up the call sites. This will only apply the flag when necessary.
+#
+# This config is just an alias to no_shorten_64_warnings and will
+# suppress a superset of warning 4267 and any 64-bit -> 32-bit implicit
+# conversions. Having both for a time means not having to go through and
+# update all references to no_size_t_to_int_warning throughout the codebase
+# atomically.
+#
+# Any new warning suppressions should use the no_shorten_64_warnings
+# config below and not this.
+#
+# TODO(jschuh): crbug.com/167187 fix this and delete this config.
+config("no_size_t_to_int_warning") {
+  configs = [ ":no_shorten_64_warnings" ]
+}
+
+# As part of re-enabling -Wconversion (see issue 588506) some code
+# will continue to generate warnings.
+# The first warning to be enabled will be -Wshorten-64-to-32.
+#
+# Code that currently generates warnings for this can include this
+# config to disable them.
+config("no_shorten_64_warnings") {
+  if (current_cpu == "x64" || current_cpu == "arm64") {
+    if (is_clang) {
+      cflags = [ "-Wno-shorten-64-to-32" ]
+    } else {
+      if (is_win) {
+        # MSVC does not have an explicit warning equivalent to
+        # -Wshorten-64-to-32 but 4267 warns for size_t -> int
+        # on 64-bit builds, so is the closest.
+        cflags = [ "/wd4267" ]
+      }
+    }
+  }
+}
+
 # Some code presumes that pointers to structures/objects are compatible
 # regardless of whether what they point to is already known to be valid.
 # gcc 4.9 and earlier had no way of suppressing this warning without
@@ -2016,11 +2122,15 @@ config("no_incompatible_pointer_warnings") {
 # Shared settings for both "optimize" and "optimize_max" configs.
 # IMPORTANT: On Windows "/O1" and "/O2" must go before the common flags.
 if (is_win) {
+  if (is_starboard) {
+    common_optimize_on_cflags = []
+  } else {
   common_optimize_on_cflags = [
     "/Ob2",  # Both explicit and auto inlining.
     "/Oy-",  # Disable omitting frame pointers, must be after /O2.
     "/Zc:inline",  # Remove unreferenced COMDAT (faster links).
   ]
+  }
   if (!is_asan) {
     common_optimize_on_cflags += [
       # Put data in separate COMDATs. This allows the linker
diff --git a/build/config/compiler/compiler.gni b/build/config/compiler/compiler.gni
index 4738ee80d307..aa5f37fd0b06 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -37,6 +37,10 @@ declare_args() {
   # The default linker everywhere else.
   use_lld = is_clang && current_os != "zos"
 
+  if (use_cobalt_customizations && is_apple) {
+    use_lld = false
+  }
+
   # If true, optimize for size.
   # Default to favoring speed over size for platforms not listed below.
   optimize_for_size =
diff --git a/build/config/dcheck_always_on.gni b/build/config/dcheck_always_on.gni
index cca3a547cd55..c14c5882ace8 100644
--- a/build/config/dcheck_always_on.gni
+++ b/build/config/dcheck_always_on.gni
@@ -5,7 +5,6 @@
 # TODO(crbug.com/1233050): Until the bug is resolved we need to include
 # gclient_args for the definition of build_with_chromium and build_overrides
 # for client overrides of that flag. The latter should go away.
-import("//build/config/gclient_args.gni")
 import("//build_overrides/build.gni")
 declare_args() {
   # Enables DCHECKs to be built-in, but to default to being non-fatal/log-only.
diff --git a/build/config/fuchsia/test/web_instance_host_capabilities.test-cmx b/build/config/fuchsia/test/web_instance_host_capabilities.test-cmx
new file mode 100644
index 000000000000..762916ba9f6d
--- /dev/null
+++ b/build/config/fuchsia/test/web_instance_host_capabilities.test-cmx
@@ -0,0 +1,8 @@
+{
+  "sandbox": {
+    "services": [
+      "fuchsia.sys.Environment",
+      "fuchsia.sys.Loader"
+    ]
+  }
+}
\ No newline at end of file
diff --git a/build/config/mac/mac_sdk.gni b/build/config/mac/mac_sdk.gni
index d3c4e3c7fdd1..db7c236d726f 100644
--- a/build/config/mac/mac_sdk.gni
+++ b/build/config/mac/mac_sdk.gni
@@ -3,7 +3,9 @@
 # found in the LICENSE file.
 
 import("//build/config/chrome_build.gni")
+if (!is_starboard) {
 import("//build/config/gclient_args.gni")
+}
 import("//build/config/mac/mac_sdk_overrides.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/rbe.gni")
diff --git a/build/config/ozone.gni b/build/config/ozone.gni
index 8bb512ad634b..00eac5ee5bd3 100644
--- a/build/config/ozone.gni
+++ b/build/config/ozone.gni
@@ -9,7 +9,7 @@ import("//build/toolchain/toolchain.gni")
 declare_args() {
   # Indicates if Ozone is enabled. Ozone is a low-level library layer for Linux
   # that does not require X11.
-  use_ozone = is_chromeos || is_fuchsia || is_linux
+  use_ozone = is_chromeos || is_fuchsia || is_linux && !is_starboard
 }
 
 declare_args() {
diff --git a/build/config/pch.gni b/build/config/pch.gni
index bc4e9e6d150b..9cb42033189f 100644
--- a/build/config/pch.gni
+++ b/build/config/pch.gni
@@ -11,5 +11,5 @@ declare_args() {
   # doing official builds.
   # On Linux it slows down the build, so don't enable it by default.
   enable_precompiled_headers =
-      !is_official_build && !(use_goma || use_remoteexec) && !is_linux
+      !is_official_build && !(use_goma || use_remoteexec) && !is_linux && !is_starboard
 }
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index c320ed8d9969..67ca2ed253f5 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -123,10 +123,12 @@ static_library("options_sources") {
   ]
   sources = [ "//build/sanitizers/sanitizer_options.cc" ]
 
+  if (!use_cobalt_customizations) {
   # Don't compile this target with any sanitizer code. It can be called from
   # the sanitizer runtimes, so instrumenting these functions could cause
   # recursive calls into the runtime if there is an error.
   configs -= [ "//build/config/sanitizers:default_sanitizer_flags" ]
+  }
 
   if (is_asan) {
     if (!defined(asan_suppressions_file)) {
diff --git a/build/config/sysroot.gni b/build/config/sysroot.gni
index dea380727e73..b81eb0055745 100644
--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -22,6 +22,10 @@ declare_args() {
   use_sysroot = current_cpu == "x86" || current_cpu == "x64" ||
                 current_cpu == "arm" || current_cpu == "arm64" ||
                 current_cpu == "mipsel" || current_cpu == "mips64el"
+
+  if (use_cobalt_customizations) {
+    use_sysroot = false
+  }
 }
 
 if (sysroot == "") {
diff --git a/build/config/ui.gni b/build/config/ui.gni
index b560f372c638..f9e6bb9a6141 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -23,6 +23,11 @@ import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/ozone.gni")
 
 declare_args() {
+  # Indicates if the UI toolkit depends on X11.
+  # Enabled by default. Can be disabled if Ozone only build is required and
+  # vice-versa.
+  use_x11 = is_linux && !is_chromecast && !is_chromeos_lacros
+
   # Indicates if Aura is enabled. Aura is a low-level windowing library, sort
   # of a replacement for GDI or GTK.
   use_aura = is_win || is_linux || is_chromeos || is_fuchsia
@@ -33,7 +38,7 @@ declare_args() {
   toolkit_views = is_mac || is_win || is_linux || is_chromeos || is_fuchsia
 
   use_glib =
-      is_linux && !is_castos &&
+      is_linux && !is_castos && !is_starboard &&
       # Avoid the need for glib when Android is building things via secondary
       # toolchains.
       target_os != "android"
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index 8a3bfbbb526e..a0a28102a789 100644
--- a/build/config/win/BUILD.gn
+++ b/build/config/win/BUILD.gn
@@ -9,7 +9,9 @@ import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/control_flow_guard.gni")
 import("//build/config/win/visual_studio_version.gni")
+if (!is_starboard) {
 import("//build/timestamp.gni")
+}
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/rbe.gni")
 import("//build/toolchain/toolchain.gni")
diff --git a/build/config/win/visual_studio_version.gni b/build/config/win/visual_studio_version.gni
index 1da479dd5eeb..5c2c27e8732c 100644
--- a/build/config/win/visual_studio_version.gni
+++ b/build/config/win/visual_studio_version.gni
@@ -2,6 +2,47 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if (is_starboard) {
+  declare_args() {
+    visual_studio_version = "14.34.31933"
+
+    # Full path to the Windows SDK, not including a backslash at the end.
+    # This value is the default location, override if you have a different
+    # installation location.
+    windows_sdk_path = "C:/Program Files (x86)/Windows Kits/10"
+
+    # Version of Windows SDK. XB1 requires >=10.0.22000.0.
+    if (current_os == "winuwp") {
+      wdk_version = "10.0.22000.0"
+    } else {
+      wdk_version = "10.0.18362.0"
+    }
+  }
+
+  if (is_docker_build) {
+    _default_visual_studio_path = "C:/BuildTools"
+  } else {
+    _default_visual_studio_path = getenv("VSINSTALLDIR")
+    if (_default_visual_studio_path == "") {
+      _default_visual_studio_path = "C:/Program Files (x86)/Microsoft Visual Studio/2022/Professional"
+    }
+  }
+
+  declare_args() {
+    # Path to Visual Studio.
+    visual_studio_path = _default_visual_studio_path
+
+    wdk_include_path = "$windows_sdk_path/include/$wdk_version"
+
+    wdk_lib_path = "$windows_sdk_path/lib/$wdk_version"
+  }
+
+  declare_args() {
+    msvc_path = "$visual_studio_path/VC/Tools/MSVC/$visual_studio_version"
+
+    llvm_clang_path = "$visual_studio_path/VC/Tools/Llvm/x64/bin"
+  }
+} else {
 declare_args() {
   # Path to Visual Studio. If empty, the default is used which is to use the
   # automatic toolchain in depot_tools. If set, you must also set the
@@ -23,6 +64,7 @@ declare_args() {
   # Version of the Windows SDK pointed to by the windows_sdk_path.
   windows_sdk_version = ""
 }
+}
 
 if (visual_studio_path == "") {
   toolchain_data =
@@ -33,7 +75,7 @@ if (visual_studio_path == "") {
   visual_studio_version = toolchain_data.vs_version
   wdk_path = toolchain_data.wdk_dir
   visual_studio_runtime_dirs = toolchain_data.runtime_dirs
-} else {
+} else if (!is_starboard) {
   assert(visual_studio_version != "",
          "You must set the visual_studio_version if you set the path")
   assert(windows_sdk_version != "",
diff --git a/build/lacros/lacros_resource_sizes.pydeps b/build/lacros/lacros_resource_sizes.pydeps
index c2437ca51252..dc6a11699674 100644
--- a/build/lacros/lacros_resource_sizes.pydeps
+++ b/build/lacros/lacros_resource_sizes.pydeps
@@ -1,5 +1,6 @@
 # Generated by running:
 #   build/print_python_deps.py --root build/lacros --output build/lacros/lacros_resource_sizes.pydeps build/lacros/lacros_resource_sizes.py
+../../third_party/catapult/third_party/six/six.py
 ../../third_party/catapult/third_party/vinn/vinn/__init__.py
 ../../third_party/catapult/third_party/vinn/vinn/_vinn.py
 ../../third_party/catapult/tracing/tracing/__init__.py
diff --git a/build/lacros/test_runner.py b/build/lacros/test_runner.py
index ab319dec62e3..856acd012e69 100755
--- a/build/lacros/test_runner.py
+++ b/build/lacros/test_runner.py
@@ -843,6 +843,16 @@ def Main():
       action='store_true',
       help='Whether to run subprocess log outputs through the asan symbolizer.')
 
+  # This is for version skew testing. The current CI/CQ builder builds
+  # an ash chrome and pass it using --ash-chrome-path. In order to use the same
+  # builder for version skew testing, we use a new argument to override
+  # the ash chrome.
+  test_parser.add_argument(
+      '--ash-chrome-path-override',
+      type=str,
+      help='The same as --ash-chrome-path. But this will override '
+      '--ash-chrome-path or --ash-chrome-version if any of these '
+      'arguments exist.')
   args = arg_parser.parse_known_args()
   if not hasattr(args[0], "func"):
     # No command specified.
diff --git a/build/toolchain/BUILD.gn b/build/toolchain/BUILD.gn
index a3bd8c58cc70..73e3a71cc21b 100644
--- a/build/toolchain/BUILD.gn
+++ b/build/toolchain/BUILD.gn
@@ -11,7 +11,7 @@ declare_args() {
   action_pool_depth = -1
 }
 
-if (current_toolchain == default_toolchain) {
+if (is_starboardized_toolchain || current_toolchain == default_toolchain) {
   if (action_pool_depth == -1 || (use_goma || use_remoteexec)) {
     action_pool_depth = exec_script("get_cpu_count.py", [], "value")
   }
diff --git a/build/toolchain/apple/toolchain.gni b/build/toolchain/apple/toolchain.gni
index 70d7c036392a..a3803e157d0b 100644
--- a/build/toolchain/apple/toolchain.gni
+++ b/build/toolchain/apple/toolchain.gni
@@ -157,7 +157,20 @@ template("single_apple_toolchain") {
       toolchain_is_component_build = is_component_build
     }
 
-    prefix = rebase_path("$clang_base_path/bin/", root_build_dir)
+    if (defined(toolchain_args.use_xcode_clang)) {
+      toolchain_uses_xcode_clang = toolchain_args.use_xcode_clang
+    } else {
+      toolchain_uses_xcode_clang = use_xcode_clang
+    }
+
+    # Supports building with the version of clang shipped with Xcode when
+    # targeting iOS by not respecting clang_base_path.
+    if (toolchain_uses_xcode_clang) {
+      prefix = invoker.bin_path
+    } else {
+      prefix = rebase_path("$clang_base_path/bin/", root_build_dir)
+    }
+
     _cc = "${prefix}clang"
     _cxx = "${prefix}clang++"
 
diff --git a/build/toolchain/cc_wrapper.gni b/build/toolchain/cc_wrapper.gni
index d70fa7f23427..577a1fbef63e 100644
--- a/build/toolchain/cc_wrapper.gni
+++ b/build/toolchain/cc_wrapper.gni
@@ -35,6 +35,46 @@ import("//build/toolchain/rbe.gni")
 declare_args() {
   # Set to "ccache", "icecc" or "distcc".  Probably doesn't work on windows.
   cc_wrapper = ""
+
+  if (is_starboard) {
+    enable_sccache = getenv("SCCACHE") == "1"
+  }
+}
+
+if (is_starboard) {
+  declare_args() {
+    # Set to false to completely ignore the cc_wrapper.
+    enable_cc_wrapper = true
+  }
+  assert(enable_cc_wrapper || cc_wrapper == "",
+         "Do not set `cc_wrapper` if you set `enable_cc_wrapper` to false.")
+}
+
+if (is_starboard && getenv("SCCACHE") == "") {
+  enable_sccache = host_os == "win" && cobalt_fastbuild
+}
+
+if (is_starboard && enable_sccache) {
+  _set_sccache_gcs_bucket = getenv("SCCACHE_GCS_BUCKET") != ""
+  _set_sccache_gcs_key_path = getenv("SCCACHE_GCS_KEY_PATH") != ""
+  _set_sccache_gcs_oauth_url = getenv("SCCACHE_GCS_OAUTH_URL") != ""
+  _set_sccache_gcs_rw_mode = getenv("SCCACHE_GCS_RW_MODE") != ""
+  _set_sccache_dir = getenv("SCCACHE_DIR") != ""
+  assert(_set_sccache_dir ||
+             (_set_sccache_gcs_bucket &&
+                  (_set_sccache_gcs_key_path || _set_sccache_gcs_oauth_url) &&
+                  _set_sccache_gcs_rw_mode),
+         "Set Sccache environment variables before use.")
+}
+
+if (is_starboard && cc_wrapper == "" && enable_cc_wrapper) {
+  # TODO(https://crbug.com/gn/273): Use sccache locally as well.
+  if (enable_sccache) {
+    cc_wrapper = "sccache"
+  } else if (host_os != "win") {
+    cc_wrapper = "ccache"
+  }
+  print("Build Accelerator: " + cc_wrapper)
 }
 
 assert(!use_goma || cc_wrapper == "",
diff --git a/build/toolchain/concurrent_links.gni b/build/toolchain/concurrent_links.gni
index e3590228e347..a10a5cd526fc 100644
--- a/build/toolchain/concurrent_links.gni
+++ b/build/toolchain/concurrent_links.gni
@@ -20,10 +20,10 @@ declare_args() {
   # links at once than we do compiles, because linking is memory-intensive.
   # The default to use varies by platform and by the amount of memory
   # available, so we call out to a script to get the right value.
-  concurrent_links = -1
+  concurrent_links = getenv("COBALT_CONCURRENT_LINKS")
 }
 
-if (concurrent_links == -1) {
+if (concurrent_links == "") {
   if (use_thin_lto) {
     _args = [ "--reserve_mem_gb=10" ]
     if (use_goma_thin_lto) {
diff --git a/build/toolchain/gcc_toolchain.gni b/build/toolchain/gcc_toolchain.gni
index ad994319cd6f..422ed5570bfd 100644
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -13,6 +13,10 @@ import("//build/toolchain/goma.gni")
 import("//build/toolchain/rbe.gni")
 import("//build/toolchain/toolchain.gni")
 
+if (use_cobalt_customizations) {
+  import("//starboard/build/config/os_definitions.gni")
+}
+
 if (is_nacl) {
   # To keep NaCl variables out of builds that don't include NaCl, all
   # variables defined in nacl/config.gni referenced here should be protected by
@@ -113,6 +117,10 @@ if (enable_resource_allowlist_generation) {
 # without sanitizers for host-side tools.
 template("single_gcc_toolchain") {
   toolchain(target_name) {
+    is_starboard_toolchain = target_name == "starboard"
+    if (!sb_is_modular || sb_is_evergreen) {
+      not_needed(["is_starboard_toolchain"])
+    }
     assert(defined(invoker.ar), "gcc_toolchain() must specify a \"ar\" value")
     assert(defined(invoker.cc), "gcc_toolchain() must specify a \"cc\" value")
     assert(defined(invoker.cxx), "gcc_toolchain() must specify a \"cxx\" value")
@@ -132,6 +140,12 @@ template("single_gcc_toolchain") {
     assert(defined(invoker.toolchain_args),
            "Toolchains must specify toolchain_args")
     invoker_toolchain_args = invoker.toolchain_args
+    if (is_starboard && !defined(invoker_toolchain_args.current_cpu)) {
+      invoker_toolchain_args.current_cpu = target_cpu
+    }
+    if (is_starboard && !defined(invoker_toolchain_args.current_os)) {
+      invoker_toolchain_args.current_os = target_os
+    }
     assert(defined(invoker_toolchain_args.current_cpu),
            "toolchain_args must specify a current_cpu")
     assert(defined(invoker_toolchain_args.current_os),
@@ -142,11 +156,13 @@ template("single_gcc_toolchain") {
     toolchain_args = {
       # Populate toolchain args from the invoker.
       forward_variables_from(invoker_toolchain_args, "*")
+      build_with_separate_cobalt_toolchain = build_with_separate_cobalt_toolchain
 
       # The host toolchain value computed by the default toolchain's setup
       # needs to be passed through unchanged to all secondary toolchains to
       # ensure that it's always the same, regardless of the values that may be
       # set on those toolchains.
+      if (!use_cobalt_customizations) {
       host_toolchain = host_toolchain
 
       # The same applies to the toolchain we use to build Rust procedural
@@ -156,6 +172,7 @@ template("single_gcc_toolchain") {
       if (!defined(invoker_toolchain_args.v8_current_cpu)) {
         v8_current_cpu = invoker_toolchain_args.current_cpu
       }
+      }
     }
 
     # When the invoker has explicitly overridden use_remoteexec, use_goma or
@@ -295,9 +312,9 @@ template("single_gcc_toolchain") {
           "\"$python_path\" ${_coverage_wrapper} " + compiler_prefix
     }
 
-    cc = compiler_prefix + invoker.cc
-    cxx = compiler_prefix + invoker.cxx
-    asm = asm_prefix + invoker.cc
+    cc = "$compiler_prefix\"${invoker.cc}\""
+    cxx = "$compiler_prefix\"${invoker.cxx}\""
+    asm = "$asm_prefix\"${invoker.cc}\""
     ar = invoker.ar
     ld = link_prefix + invoker.ld
     if (defined(invoker.readelf)) {
@@ -381,6 +398,12 @@ template("single_gcc_toolchain") {
     # Object files go in this directory.
     object_subdir = "{{target_out_dir}}/{{label_name}}"
 
+    if (defined(invoker.tail_lib_dependencies)) {
+      tail_lib_dependencies = " " + invoker.tail_lib_dependencies
+    } else {
+      tail_lib_dependencies = ""
+    }
+
     tool("cc") {
       depfile = "{{output}}.d"
       precompiled_header_type = "gcc"
@@ -402,7 +425,16 @@ template("single_gcc_toolchain") {
     tool("asm") {
       # For GCC we can just use the C compiler to compile assembly.
       depfile = "{{output}}.d"
-      command = "$asm $md -MF $depfile ${rebuild_string}{{defines}} {{include_dirs}} {{asmflags}}${extra_asmflags} -c {{source}} -o {{output}}"
+
+      # TODO(b/206642994): see if we can remove this condition. It's needed for
+      # now to add cflags for evergreen platforms but we haven't yet decided
+      # whether cflags should be added here for all platforms.
+      if (!is_starboard_toolchain && is_starboard && sb_is_modular) {
+        command = "$asm $md -MF $depfile ${rebuild_string}{{defines}} {{include_dirs}} {{cflags}} {{asmflags}}${extra_asmflags} -c {{source}} -o {{output}}"
+      } else {
+        command = "$asm $md -MF $depfile ${rebuild_string}{{defines}} {{include_dirs}} {{asmflags}}${extra_asmflags} -c {{source}} -o {{output}}"
+      }
+
       depsformat = "gcc"
       description = "ASM {{output}}"
       outputs = [ "$object_subdir/{{source_name_part}}.o" ]
@@ -637,7 +669,17 @@ template("single_gcc_toolchain") {
         start_group_flag = "-Wl,--start-group"
         end_group_flag = "-Wl,--end-group "
       }
-      link_command = "$ld {{ldflags}}${extra_ldflags} -o \"$unstripped_outfile\" $start_group_flag @\"$rspfile\" {{solibs}} $end_group_flag {{libs}} {{rlibs}}"
+
+      if (using_old_compiler) {
+        link_command = "$ld {{ldflags}}${extra_ldflags} -o \"$unstripped_outfile\" $start_group_flag @\"$rspfile\" {{solibs}} $end_group_flag {{libs}} $tail_lib_dependencies"
+      } else {
+        # Windows based platform modular builds dont link directly with the shared library.
+        if (is_host_win && sb_is_modular) {
+          link_command = "$ld {{ldflags}}${extra_ldflags} {{libs}} -o \"$unstripped_outfile\" $start_group_flag @\"$rspfile\"  $end_group_flag$tail_lib_dependencies"
+        } else {
+          link_command = "$ld {{ldflags}}${extra_ldflags} {{libs}} -o \"$unstripped_outfile\" $start_group_flag @\"$rspfile\" {{solibs}} $end_group_flag$tail_lib_dependencies"
+        }
+      }
 
       # Generate a map file to be used for binary size analysis.
       # Map file adds ~10% to the link time on a z620.
@@ -858,6 +900,9 @@ template("gcc_toolchain") {
 # actually just be doing a native compile. The invoker can optionally override
 # use_gold too.
 template("clang_toolchain") {
+  if (is_starboard) {
+    clang_base_path = invoker.clang_base_path
+  }
   gcc_toolchain(target_name) {
     _path = "$clang_base_path/bin"
     _is_path_absolute = get_path_info(_path, "abspath") == _path
diff --git a/build/toolchain/ios/compile_xcassets.py b/build/toolchain/ios/compile_xcassets.py
index a62b96a4d7b6..418bde507b6c 100644
--- a/build/toolchain/ios/compile_xcassets.py
+++ b/build/toolchain/ios/compile_xcassets.py
@@ -46,6 +46,15 @@ def FixAbsolutePathInLine(line, relative_paths):
   return relative_path + line[len(absolute_path):]
 
 
+def FixAbsolutePathInLine(line, relative_paths):
+  """Fix absolute paths present in |line| to relative paths."""
+  absolute_path = line.split(':')[0]
+  relative_path = relative_paths.get(absolute_path, absolute_path)
+  if absolute_path == relative_path:
+    return line
+  return relative_path + line[len(absolute_path):]
+
+
 def FilterCompilerOutput(compiler_output, relative_paths):
   """Filers actool compilation output.
 
diff --git a/build/toolchain/win/msvc_toolchain.gni b/build/toolchain/win/msvc_toolchain.gni
new file mode 100644
index 000000000000..01c3ebbdcf69
--- /dev/null
+++ b/build/toolchain/win/msvc_toolchain.gni
@@ -0,0 +1,244 @@
+# Copyright 2021 The Cobalt Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+assert(use_cobalt_customizations)
+
+import("//build/toolchain/cc_wrapper.gni")
+import("//build/toolchain/toolchain.gni")
+
+tool_wrapper_path = rebase_path("tool_wrapper.py", root_build_dir)
+
+template("msvc_toolchain") {
+  # Write the environment variables file in the out directory.
+  required_environment_variables = [
+    "SYSTEMROOT",
+    "TEMP",
+    "TMP",
+  ]
+  optional_environment_variables = [
+    "INCLUDE",
+    "LIB",
+    "PATH",
+    "PATHEXT",
+    "XEDK",  # TODO: What does this do?
+    "IS_DOCKER",  # needed for ninja to invoke docker-specific logic
+    "IS_CI",  # needed for ninja to exclude some logic on GKE
+
+    # The remaining variables should be explicitly enumerated.
+    # "cell_.*",
+    # "sn_.*",
+    # "sce_.*",
+  ]
+
+  environment_key_value_pairs = []
+
+  foreach(key, required_environment_variables) {
+    value = getenv(key)
+    assert(value != "", "Envionment variable $key is required to be set.")
+    environment_key_value_pairs += [ "$key=$value" ]
+  }
+  foreach(key, optional_environment_variables) {
+    value = getenv(key)
+    if (value != "") {
+      environment_key_value_pairs += [ "$key=$value" ]
+    }
+  }
+
+  nul = "$0x00"
+  env_block = string_join(nul, environment_key_value_pairs) + nul
+
+  # If the default toolchain shares its name with another toolchain,
+  # there could be a deadlock where both toolchains try to write to the same environment file.
+  # See b/297227714 for more context.
+  write_file("$root_build_dir/$target_name/environment.$target_cpu", env_block)
+
+  toolchain(target_name) {
+    # When invoking this toolchain not as the default one, these args will be
+    # passed to the build. They are ignored when this is the default toolchain.
+    assert(defined(invoker.toolchain_args))
+    toolchain_args = {
+      if (defined(invoker.toolchain_args)) {
+        forward_variables_from(invoker.toolchain_args, "*")
+      }
+    }
+
+    # Make these apply to all tools below.
+    lib_switch = ""
+    lib_dir_switch = "/LIBPATH:"
+
+    # Object files go in this directory.
+    object_subdir = "{{target_out_dir}}/{{label_name}}"
+
+    env = "$target_name/environment.$target_cpu"
+    cl = invoker.cl
+    lib = invoker.lib
+    link = invoker.link
+    asm = invoker.asm
+
+    env_wrapper = "ninja -t msvc -e $env -- "  # Note trailing space.
+
+    # TODO(https://crbug.com/gn/273): Always use sccache for cc and cxx tools.
+    if (cc_wrapper != "") {
+      cl_prefix = "${cc_wrapper} "  # Note trailing space.
+    } else {
+      cl_prefix = env_wrapper
+    }
+
+    sys_include_flags = ""
+
+    tool("cc") {
+      precompiled_header_type = "msvc"
+      pdbname = "{{target_out_dir}}/{{label_name}}_c.pdb"
+
+      # Label names may have spaces in them so the pdbname must be quoted. The
+      # source and output don't need to be quoted because GN knows they're a
+      # full file name and will quote automatically when necessary.
+      depsformat = "msvc"
+      description = "CC {{output}}"
+      outputs = [ "$object_subdir/{{source_name_part}}.obj" ]
+
+      command = "$cl_prefix\"$cl\" /nologo /showIncludes $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
+    }
+
+    tool("cxx") {
+      precompiled_header_type = "msvc"
+
+      # The PDB name needs to be different between C and C++ compiled files.
+      pdbname = "{{target_out_dir}}/{{label_name}}_cc.pdb"
+
+      # See comment in CC tool about quoting.
+      depsformat = "msvc"
+      description = "CXX {{output}}"
+      outputs = [ "$object_subdir/{{source_name_part}}.obj" ]
+
+      command = "$cl_prefix\"$cl\" /nologo /showIncludes $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
+    }
+
+    tool("asm") {
+      description = "ASM {{output}}"
+      outputs = [ "$object_subdir/{{source_name_part}}.obj" ]
+      command = "$env_wrapper$asm /nologo /Fo{{output}} /c {{defines}} {{include_dirs}} {{asmflags}} {{source}}"
+    }
+
+    sys_lib_flags = "${invoker.sys_lib_flags} "  # Note trailing space.
+
+    tool("alink") {
+      rspfile = "{{output}}.rsp"
+      command = "$env_wrapper$lib /ignore:4221 /OUT:{{output}} /nologo ${sys_lib_flags}{{arflags}} @$rspfile"
+      description = "LIB {{output}}"
+      outputs = [
+        # Ignore {{output_extension}} and always use .lib, there's no reason to
+        # allow targets to override this extension on Windows.
+        "{{output_dir}}/{{target_output_name}}.lib",
+      ]
+      default_output_extension = ".lib"
+      default_output_dir = "{{target_out_dir}}"
+
+      # The use of inputs_newline is to work around a fixed per-line buffer
+      # size in the linker.
+      rspfile_content = "{{inputs_newline}}"
+    }
+
+    # TODO(b/217794556): All following linker tools should list the PDB file in
+    # their outputs. It has been removed as it is not always generated, and
+    # ninja will treat the missing file as a dirty edge.
+    tool("solink") {
+      # E.g. "foo.dll":
+      dllname = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+      libname = "${dllname}.lib"  # e.g. foo.dll.lib
+      pdbname = "${dllname}.pdb"
+      rspfile = "${dllname}.rsp"
+      pool = "//build/toolchain:link_pool($default_toolchain)"
+
+      command = "$env_wrapper$link /OUT:$dllname /nologo ${sys_lib_flags}/IMPLIB:$libname /DLL /PDB:$pdbname @$rspfile"
+
+      default_output_extension = ".dll"
+      default_output_dir = "{{root_out_dir}}"
+      description = "LINK(DLL) {{output}}"
+      outputs = [
+        dllname,
+        libname,
+      ]
+      link_output = libname
+      depend_output = libname
+      runtime_outputs = [ dllname ]
+
+      # Since the above commands only updates the .lib file when it changes, ask
+      # Ninja to check if the timestamp actually changed to know if downstream
+      # dependencies should be recompiled.
+      restat = true
+
+      # The use of inputs_newline is to work around a fixed per-line buffer
+      # size in the linker.
+      rspfile_content = "{{libs}} {{solibs}} {{inputs_newline}} {{ldflags}}"
+    }
+
+    tool("solink_module") {
+      # E.g. "foo.dll":
+      dllname = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+      pdbname = "${dllname}.pdb"
+      rspfile = "${dllname}.rsp"
+      pool = "//build/toolchain:link_pool($default_toolchain)"
+
+      command = "$env_wrapper$link /OUT:$dllname /nologo ${sys_lib_flags}/DLL /PDB:$pdbname @$rspfile"
+
+      default_output_extension = ".dll"
+      default_output_dir = "{{root_out_dir}}"
+      description = "LINK_MODULE(DLL) {{output}}"
+      outputs = [ dllname ]
+      runtime_outputs = outputs
+
+      # The use of inputs_newline is to work around a fixed per-line buffer
+      # size in the linker.
+      rspfile_content = "{{libs}} {{solibs}} {{inputs_newline}} {{ldflags}}"
+    }
+
+    tool("link") {
+      exename = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+      pdbname = "$exename.pdb"
+      rspfile = "$exename.rsp"
+      pool = "//build/toolchain:link_pool($default_toolchain)"
+
+      command = "$env_wrapper$link /OUT:$exename /nologo ${sys_lib_flags} /PDB:$pdbname @$rspfile"
+
+      default_output_extension = ".exe"
+      default_output_dir = "{{root_out_dir}}"
+      description = "LINK {{output}}"
+      outputs = [ exename ]
+      runtime_outputs = outputs
+
+      # The use of inputs_newline is to work around a fixed per-line buffer
+      # size in the linker.
+      rspfile_content = "{{inputs_newline}} {{libs}} {{solibs}} {{ldflags}}"
+    }
+
+    # These two are really entirely generic, but have to be repeated in
+    # each toolchain because GN doesn't allow a template to be used here.
+    # See //build/toolchain/toolchain.gni for details.
+    tool("stamp") {
+      command = stamp_command
+      description = stamp_description
+      pool = "//build/toolchain:action_pool($default_toolchain)"
+    }
+    tool("copy") {
+      command = copy_command
+      description = copy_description
+      pool = "//build/toolchain:action_pool($default_toolchain)"
+    }
+
+    tool("action") {
+      pool = "//build/toolchain:action_pool($default_toolchain)"
+    }
+  }
+}
diff --git a/build/win/BUILD.gn b/build/win/BUILD.gn
index 864581851519..cfb8d162f7d3 100644
--- a/build/win/BUILD.gn
+++ b/build/win/BUILD.gn
@@ -6,6 +6,10 @@ import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/manifest.gni")
 
+if (is_starboard) {
+  # Never use a manifest in Starboard.
+  group("default_exe_manifest") {}
+} else {
 # Depending on this target will cause the manifests for Chrome's default
 # Windows and common control compatibility and elevation for executables.
 windows_manifest("default_exe_manifest") {
@@ -15,11 +19,13 @@ windows_manifest("default_exe_manifest") {
     default_compatibility_manifest,
   ]
 }
+}
 
 if (is_win) {
   assert(host_os != "mac" || target_cpu != "x86",
          "Windows cross-builds from Mac must be 64-bit.")
 
+  if (!is_starboard) {
   action("copy_cdb_to_output") {
     script = "//build/win/copy_cdb_to_output.py"
     inputs = [
@@ -41,6 +47,7 @@ if (is_win) {
       current_cpu,
     ]
   }
+  }
 
   group("runtime_libs") {
     # These are needed for any tests that need to decode stacks.
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index c69e1f649542..b5c8bd720371 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -13,12 +13,14 @@
 # limitations under the License.
 
 build_with_chromium = false
+is_nacl_nonsfi = false
 
 declare_args() {
   # Android 32-bit non-component, non-clang builds cannot have symbol_level=2
   # due to 4GiB file size limit, see https://crbug.com/648948.
   # Set this flag to true to skip the assertion.
   ignore_elf32_limitations = false
+  is_chromecast = false
 }
 
 enable_java_templates = false
diff --git a/build_overrides/crypto.gni b/build_overrides/crypto.gni
new file mode 100644
index 000000000000..ff40f763c88d
--- /dev/null
+++ b/build_overrides/crypto.gni
@@ -0,0 +1,17 @@
+# Copyright 2024 The Cobalt Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+declare_args() {
+  use_nss_certs = false
+}
diff --git a/crypto/BUILD.gn b/crypto/BUILD.gn
index a8d6f2b7a30b..4208bb092d09 100644
--- a/crypto/BUILD.gn
+++ b/crypto/BUILD.gn
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/crypto.gni")
+import("//build_overrides/crypto.gni")
 import("//testing/test.gni")
 
 component("crypto") {
diff --git a/starboard/build/config/BUILDCONFIG.gn b/starboard/build/config/BUILDCONFIG.gn
index f17850279d29..bf7076396711 100644
--- a/starboard/build/config/BUILDCONFIG.gn
+++ b/starboard/build/config/BUILDCONFIG.gn
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-_is_python2 = exec_script("//build/util/is_python2.py", [], "json")
+_is_python2 = exec_script("//starboard/build/util/is_python2.py", [], "json")
 assert(!_is_python2, "`python` must resolve to Python 3 when building with GN.")
 
 declare_args() {
diff --git a/starboard/build/util/is_python2.py b/starboard/build/util/is_python2.py
new file mode 100644
index 000000000000..fbfa431ae76f
--- /dev/null
+++ b/starboard/build/util/is_python2.py
@@ -0,0 +1,10 @@
+# Copyright 2024 The Cobalt Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Script for checking if we're running Python 2 or 3."""
+
+from __future__ import print_function
+
+import sys
+
+print("true" if sys.version_info.major == 2 else "false")
diff --git a/starboard/linux/x64x11/clang/3.9/platform_configuration/BUILD.gn b/starboard/linux/x64x11/clang/3.9/platform_configuration/BUILD.gn
index b2037fdd0b9c..6d6b6bd9f81f 100644
--- a/starboard/linux/x64x11/clang/3.9/platform_configuration/BUILD.gn
+++ b/starboard/linux/x64x11/clang/3.9/platform_configuration/BUILD.gn
@@ -97,6 +97,9 @@ config("compiler_flags") {
 
     # Suppress warnings in libjpeg
     "-Wno-shift-negative-value",
+
+    # Let older Clangs ignore newer Clangs' warnings.
+    "-Wno-unknown-warning-option",
   ]
 
   cflags_c += [
diff --git a/third_party/angle/gni/angle.gni b/third_party/angle/gni/angle.gni
index 75ea0c5ae4ad..26d0cff78002 100644
--- a/third_party/angle/gni/angle.gni
+++ b/third_party/angle/gni/angle.gni
@@ -38,7 +38,7 @@ if (angle_has_build) {
 
 if (build_with_chromium) {
   import("//ui/ozone/ozone.gni")
-} else {
+} else if (!defined(ozone_platform_gbm)) {
   declare_args() {
     ozone_platform_gbm = false
   }