diff --git a/appveyor.yml b/appveyor.yml
index 641b861165a..dee402b8c46 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -32,6 +32,8 @@ environment:
     C:\\Program Files\\AppVeyor\\BuildAgent;"
   CC: "clang-cl.exe"
   CXX: "clang-cl.exe"
+  LINKER: "lld-link.exe"
+  PYTHON3: "C:\\Python37-x64\\python.exe"
 
   matrix:
   - TARGET: nightly-x86_64-pc-windows-msvc
diff --git a/etc/ci/buildbot_steps.yml b/etc/ci/buildbot_steps.yml
index 5cbeac6cc0f..eb7593d44fa 100644
--- a/etc/ci/buildbot_steps.yml
+++ b/etc/ci/buildbot_steps.yml
@@ -6,6 +6,10 @@ env:
   GST_DEBUG: '3'
 
 mac-rel-wpt2:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -13,6 +17,10 @@ mac-rel-wpt2:
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
 
 mac-rel-wpt3:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -20,6 +28,10 @@ mac-rel-wpt3:
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --tracker-api default --reporter-api default
 
 mac-rel-wpt4:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -29,6 +41,10 @@ mac-rel-wpt4:
   - ./mach filter-intermittents async-parsing-errorsummary.log --log-intermittents async-parsing-intermittents.log --log-filteredsummary filtered-async-parsing-errorsummary.log --tracker-api default --reporter-api default
 
 mac-rel-css1:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -37,6 +53,10 @@ mac-rel-css1:
   - bash ./etc/ci/lockfile_changed.sh
 
 mac-rel-css2:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -45,8 +65,8 @@ mac-rel-css2:
 
 linux-rel-intermittent:
  env:
-   CC: gcc-5
-   CXX: g++-5
+   CC: clang-4.0
+   CXX: clang++-4.0
  commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
@@ -57,8 +77,8 @@ linux-rel-intermittent:
 
 linux-rel-nogate:
  env:
-   CC: gcc-5
-   CXX: g++-5
+   CC: clang-4.0
+   CXX: clang++-4.0
  commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
@@ -69,6 +89,10 @@ linux-rel-nogate:
   - ./etc/ci/clean_build_artifacts.sh
 
 mac-rel-intermittent:
+ env:
+   CC: /usr/local/opt/llvm/bin/clang
+   CXX: /usr/local/opt/llvm/bin/clang++
+ commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
   - env PKG_CONFIG_PATH=/usr/local/opt/zlib/lib/pkgconfig ./mach build --release
@@ -78,8 +102,8 @@ linux-rel-wpt:
   env:
     CCACHE: sccache
     RUSTC_WRAPPER: sccache
-    CC: gcc-5
-    CXX: g++-5
+    CC: clang-4.0
+    CXX: clang++-4.0
   commands:
     - ./mach clean-nightlies --keep 3 --force
     - ./mach clean-cargo-cache --keep 3 --force
@@ -97,8 +121,8 @@ linux-rel-css:
   env:
     CCACHE: sccache
     RUSTC_WRAPPER: sccache
-    CC: gcc-5
-    CXX: g++-5
+    CC: clang-4.0
+    CXX: clang++-4.0
   commands:
     - ./mach clean-nightlies --keep 3 --force
     - ./mach clean-cargo-cache --keep 3 --force
@@ -111,8 +135,8 @@ linux-rel-css:
 
 linux-nightly:
  env:
-  CC: gcc-5
-  CXX: g++-5
+  CC: clang-4.0
+  CXX: clang++-4.0
  commands:
   - ./mach clean-nightlies --keep 3 --force
   - ./mach clean-cargo-cache --keep 3 --force
@@ -154,7 +178,7 @@ magicleap-nightly:
 arm64:
  env:
   AR: /usr/bin/aarch64-linux-gnu-ar
-  AS: /usr/bin/aarch64-linux-gnu-as
+  AS: /usr/bin/aarch64-linux-gnu-gcc
   CC: /usr/bin/clang-4.0
   CFLAGS: --target=aarch64-unknown-linux-gnu -I/usr/aarch64-linux-gnu/include/ -fuse-ld=/home/servo/buildbot/slave/arm64/build/support/arm64/fake-ld.sh -Wno-error=unused-command-line-argument
   CPP: /usr/bin/clang-4.0 -E
@@ -162,10 +186,10 @@ arm64:
   CXX: /usr/bin/clang++-4.0
   CXXFLAGS: --target=aarch64-unknown-linux-gnu -I/usr/aarch64-linux-gnu/include/ -I/usr/aarch64-linux-gnu/include/c++/4.8.4/aarch64-linux-gnu -fuse-ld=/home/servo/buildbot/slave/arm64/build/support/arm64/fake-ld.sh -Wno-error=unused-command-line-argument
   EXPAT_LIB_DIR: /usr/lib/aarch64-linux-gnu
-  HOST_CC: gcc-5
+  HOST_CC: clang-4.0
   # test-tidy doesn't like empty strings
   HOST_CFLAGS: --target=x86_64-unknown-linux-gnu
-  HOST_CXX: g++-5
+  HOST_CXX: clang++-4.0
   # test-tidy doesn't like empty strings
   HOST_CXXFLAGS: --target=x86_64-unknown-linux-gnu
   LD: /usr/bin/aarch64-linux-gnu-ld
diff --git a/etc/taskcluster/decision_task.py b/etc/taskcluster/decision_task.py
index 73899105eca..fe5931ff07d 100644
--- a/etc/taskcluster/decision_task.py
+++ b/etc/taskcluster/decision_task.py
@@ -133,6 +133,10 @@ windows_build_env = {
         "LIB": "%HOMEDRIVE%%HOMEPATH%\\gst\\gstreamer\\1.0\\x86_64\\lib;%LIB%",
         "GSTREAMER_1_0_ROOT_X86_64": "%HOMEDRIVE%%HOMEPATH%\\gst\\gstreamer\\1.0\\x86_64\\",
     },
+    "all": {
+        "PYTHON3": "%HOMEDRIVE%%HOMEPATH%\\python3\\python.exe",
+        "LINKER": "lld-link.exe",
+    },
 }
 
 windows_sparse_checkout = [
@@ -256,6 +260,8 @@ def android_arm32_dev_from_macos():
         macos_build_task("Dev build (macOS)")
         .with_treeherder("Android ARMv7")
         .with_script("""
+            export HOST_CC="$(brew --prefix llvm)/bin/clang"
+            export HOST_CXX="$(brew --prefix llvm)/bin/clang++"
             ./mach bootstrap-android --accept-all-licences --build
             ./mach build --android --dev
         """)
@@ -685,9 +691,18 @@ def windows_build_task(name, package=True, arch="x86_64"):
     task = (
         windows_task(name)
         .with_max_run_time_minutes(60)
-        .with_env(**build_env, **windows_build_env[arch])
+        .with_env(
+            **build_env,
+            **windows_build_env[arch],
+            **windows_build_env["all"]
+        )
         .with_repo(sparse_checkout=windows_sparse_checkout)
         .with_python2()
+        .with_directory_mount(
+            "https://www.python.org/ftp/python/3.7.3/python-3.7.3-embed-amd64.zip",
+            sha256="6de14c9223226cf0cd8c965ecb08c51d62c770171a256991b4fddc25188cfa8e",
+            path="python3",
+        )
         .with_rustup()
         .with_repacked_msi(
             url=("https://gstreamer.freedesktop.org/data/pkg/windows/" +
diff --git a/python/servo/build_commands.py b/python/servo/build_commands.py
index abe4e1003f9..cc8eb40c48a 100644
--- a/python/servo/build_commands.py
+++ b/python/servo/build_commands.py
@@ -309,6 +309,12 @@ class MachCommands(CommandBase):
             env["CC"] = "clang-cl.exe"
             env["CXX"] = "clang-cl.exe"
 
+        host = host_triple()
+        if 'apple-darwin' in host and (not target or target == host):
+            if 'CXXFLAGS' not in env:
+                env['CXXFLAGS'] = ''
+            env["CXXFLAGS"] += "-mmacosx-version-min=10.10"
+
         if android:
             if "ANDROID_NDK" not in env:
                 print("Please set the ANDROID_NDK environment variable.")
@@ -550,7 +556,7 @@ class MachCommands(CommandBase):
 
             # The toolchain commands
             env.setdefault("AR", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-ar"))
-            env.setdefault("AS", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-as"))
+            env.setdefault("AS", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-gcc"))
             env.setdefault("CC", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-clang"))
             env.setdefault("CPP", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-clang -E"))
             env.setdefault("CXX", path.join(env["ANDROID_TOOLCHAIN_DIR"], "bin", "aarch64-linux-android-clang++"))
@@ -563,8 +569,8 @@ class MachCommands(CommandBase):
             # Undo all of that when compiling build tools for the host
             env.setdefault("HOST_CFLAGS", "")
             env.setdefault("HOST_CXXFLAGS", "")
-            env.setdefault("HOST_CC", "gcc")
-            env.setdefault("HOST_CXX", "g++")
+            env.setdefault("HOST_CC", "/usr/local/opt/llvm/bin/clang")
+            env.setdefault("HOST_CXX", "/usr/local/opt/llvm/bin/clang++")
             env.setdefault("HOST_LD", "ld")
 
             # Some random build configurations