diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 28ca829ecea..88131cccaf9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -22,6 +22,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-3.0
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - run: bundle install --local || bundle install
       - run: bundle exec rake rubocop
 
@@ -37,6 +39,8 @@ jobs:
       image: flavorjones/nokogiri-test:${{matrix.image}}
     steps:
       - uses: actions/checkout@v1 # v1 because of https://github.com/actions/checkout/issues/334
+        with:
+          submodules: true
       - run: bundle install --local || bundle install
       - run: bundle exec rake compile -- --${{matrix.sys}}-system-libraries
       - run: bundle exec rake test
@@ -53,6 +57,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-${{matrix.ruby}}
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         if: matrix.sys == 'disable'
         with:
@@ -74,6 +80,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-${{matrix.ruby}}
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         if: matrix.sys == 'disable'
         with:
@@ -94,6 +102,8 @@ jobs:
       image: flavorjones/nokogiri-test:alpine
     steps:
       - uses: actions/checkout@v1 # v1 because of https://github.com/actions/checkout/issues/334
+        with:
+          submodules: true
       # skip cache because of https://github.com/actions/checkout/issues/334
       - run: bundle install --local || bundle install
       - run: bundle exec rake compile -- --${{matrix.sys}}-system-libraries
@@ -110,6 +120,8 @@ jobs:
       image: flavorjones/nokogiri-test:alpine
     steps:
       - uses: actions/checkout@v1 # v1 because of https://github.com/actions/checkout/issues/334
+        with:
+          submodules: true
       # skip cache because of https://github.com/actions/checkout/issues/334
       - run: bundle install --local || bundle install
       - run: bundle exec rake compile -- --${{matrix.sys}}-system-libraries
@@ -129,6 +141,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-${{matrix.ruby}}
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         if: matrix.sys == 'disable'
         with:
@@ -152,6 +166,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-${{matrix.ruby}}
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         if: matrix.sys == 'disable'
         with:
@@ -171,6 +187,8 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{matrix.ruby}}
@@ -197,6 +215,8 @@ jobs:
           git config --system core.autocrlf false
           git config --system core.eol lf
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: MSP-Greg/setup-ruby-pkgs@v1
         with:
           ruby-version: "${{matrix.ruby}}"
@@ -219,6 +239,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{matrix.ruby}}
diff --git a/.github/workflows/gem-install.yml b/.github/workflows/gem-install.yml
index 1c290908373..b33cd65ad2d 100644
--- a/.github/workflows/gem-install.yml
+++ b/.github/workflows/gem-install.yml
@@ -23,6 +23,8 @@ jobs:
       image: "larskanis/rake-compiler-dock-mri-x86_64-linux:1.1.0"
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         with:
           path: ports/archives
@@ -44,6 +46,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: MSP-Greg/setup-ruby-pkgs@v1
         with:
           ruby-version: "${{matrix.ruby}}"
@@ -63,6 +67,8 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "3.0"
@@ -81,6 +87,8 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: MSP-Greg/setup-ruby-pkgs@v1
         with:
           ruby-version: "3.0"
@@ -106,6 +114,8 @@ jobs:
       image: "larskanis/rake-compiler-dock-mri-${{matrix.plat}}:1.1.0"
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         with:
           path: ports/archives
@@ -126,6 +136,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{matrix.ruby}}"
@@ -144,6 +156,8 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{matrix.ruby}}"
@@ -181,6 +195,8 @@ jobs:
       image: "larskanis/rake-compiler-dock-jruby:1.1.0"
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - run: ./scripts/test-gem-build gems java
       - uses: actions/upload-artifact@v2
         with:
@@ -197,6 +213,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{matrix.ruby}}"
diff --git a/.github/workflows/truffle.yml b/.github/workflows/truffle.yml
index c32231d5749..f2fe9265814 100644
--- a/.github/workflows/truffle.yml
+++ b/.github/workflows/truffle.yml
@@ -22,6 +22,8 @@ jobs:
       image: flavorjones/nokogiri-test:truffle-nightly
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: actions/cache@v2
         with:
           path: ports/archives
diff --git a/.github/workflows/upstream.yml b/.github/workflows/upstream.yml
index fb740f517b3..3433bfa681d 100644
--- a/.github/workflows/upstream.yml
+++ b/.github/workflows/upstream.yml
@@ -24,6 +24,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-3.0
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - name: Setup libxml2
         run: |
           git clone --depth=1 https://gitlab.gnome.org/GNOME/libxml2
@@ -47,6 +49,8 @@ jobs:
       image: flavorjones/nokogiri-test:mri-3.0
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - name: Setup libxml2
         run: |
           git clone --depth=1 https://gitlab.gnome.org/GNOME/libxml2
@@ -77,6 +81,8 @@ jobs:
           git config --system core.autocrlf false
           git config --system core.eol lf
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: MSP-Greg/setup-ruby-pkgs@ucrt
         with:
           ruby-version: "head"
@@ -101,6 +107,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: MSP-Greg/setup-ruby-pkgs@v1
         with:
           ruby-version: "head"
@@ -118,6 +126,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: true
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "jruby-head"
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 00000000000..d88e8b3af76
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,4 @@
+[submodule "test/html5lib-tests"]
+	path = test/html5lib-tests
+	url = https://github.com/stevecheckoway/html5lib-tests.git
+	branch = all-error-fixes
diff --git a/appveyor.yml b/appveyor.yml
index 872838a6871..c10942c99a7 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -22,6 +22,7 @@ install:
   - ruby --version
   - gem --version
   - gem install bundler --conservative
+  - git submodule update --init
   - bundle config --local path vendor/bundle
   - bundle install
   - IF DEFINED INSTALL_PACKAGES ( ridk exec pacman --noconfirm --needed --sync %INSTALL_PACKAGES% )
diff --git a/nokogumbo-import/appveyor.yml b/nokogumbo-import/appveyor.yml
deleted file mode 100644
index 4e54cbb029a..00000000000
--- a/nokogumbo-import/appveyor.yml
+++ /dev/null
@@ -1,38 +0,0 @@
-image: Visual Studio 2019
-
-version: 1.0.{build}-{branch}
-
-cache:
-  - vendor/bundle
-
-environment:
-  NO_GUMBO_TESTS: true
-  matrix:
-    - RUBY_VERSION: 25
-    - RUBY_VERSION: 25-x64
-    - RUBY_VERSION: 26
-    - RUBY_VERSION: 26-x64
-    - RUBY_VERSION: 27
-    - RUBY_VERSION: 27-x64
-    - RUBY_VERSION: 30
-    - RUBY_VERSION: 30-x64
-
-install:
-  - set PATH=C:\Ruby%RUBY_VERSION%\bin;C:\msys64\usr\bin;%PATH%
-  - set RAKEOPT=-rdevkit
-  - ruby --version
-  - gem --version
-  - bundle --version
-  - bundle install --path vendor/bundle
-  - bundle exec nokogiri --version
-  - git clone --depth 1 --branch all-error-fixes --single-branch https://github.com/stevecheckoway/html5lib-tests.git test/html5lib-tests
-  #- git clone --depth 1 --branch master --single-branch https://github.com/html5lib/html5lib-tests.git test/html5lib-tests
-
-build: off
-
-test_script:
-  - bash scripts/ci-package-test.sh
-
-# Uncomment to enable RDP access to the builder.
-# on_finish:
-#   - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
diff --git a/rakelib/check-manifest.rake b/rakelib/check-manifest.rake
index c8e9f4fcb51..ff4c7e7afb1 100644
--- a/rakelib/check-manifest.rake
+++ b/rakelib/check-manifest.rake
@@ -33,6 +33,7 @@ task :check_manifest do
     .cross_rubies
     .editorconfig
     .gitignore
+    .gitmodules
     .yardopts
     CHANGELOG.md
     CODE_OF_CONDUCT.md
diff --git a/test/html5lib-tests b/test/html5lib-tests
new file mode 160000
index 00000000000..e379d7a17b1
--- /dev/null
+++ b/test/html5lib-tests
@@ -0,0 +1 @@
+Subproject commit e379d7a17b18cd1bb57aec4f62edec67578d294a