diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index b40237f82802d..d99d7b32fff69 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -36,7 +36,7 @@ jobs:
   #
   test-with-mysql:
     name: PHP ${{ matrix.php }}
-    uses: WordPress/wordpress-develop/.github/workflows/reusable-phpunit-tests.yml@trunk
+    uses: costdev/wordpress-develop/.github/workflows/reusable-phpunit-tests.yml@add-html5lib-tests-to-phpunit-workflows
     permissions:
       contents: read
     secrets: inherit
@@ -51,6 +51,7 @@ jobs:
         tests-domain: [ 'example.org' ]
         multisite: [ false, true ]
         memcached: [ false ]
+        html5lib-tests: [ false ]
 
         include:
           # Include jobs for PHP 7.4 with memcached.
@@ -92,6 +93,42 @@ jobs:
             multisite: false
             memcached: false
             report: true
+          # Include html5lib tests.
+          - os: ubuntu-latest
+            php: '7.2'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '7.3'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '7.4'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '8.0'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '8.1'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '8.2'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
+          - os: ubuntu-latest
+            php: '8.3'
+            db-version: '5.7'
+            multisite: false
+            html5lib-tests: true
     with:
       os: ${{ matrix.os }}
       php: ${{ matrix.php }}
@@ -99,6 +136,7 @@ jobs:
       db-version: ${{ matrix.db-version }}
       multisite: ${{ matrix.multisite }}
       memcached: ${{ matrix.memcached }}
+      html5lib-tests: ${{ matrix.html5lib-tests }}
       phpunit-config: ${{ matrix.multisite && 'tests/phpunit/multisite.xml' || 'phpunit.xml.dist' }}
       tests-domain: ${{ matrix.tests-domain }}
       report: ${{ matrix.report || false }}
diff --git a/.github/workflows/reusable-phpunit-tests.yml b/.github/workflows/reusable-phpunit-tests.yml
index 60949915e4d1c..5f7aa2ef2b798 100644
--- a/.github/workflows/reusable-phpunit-tests.yml
+++ b/.github/workflows/reusable-phpunit-tests.yml
@@ -35,6 +35,11 @@ on:
         required: false
         type: 'boolean'
         default: false
+      html5lib-tests:
+        description: 'Whether to run the html5lib tests'
+        required: false
+        type: 'boolean'
+        default: false
       phpunit-config:
         description: 'The PHPUnit configuration file to use'
         required: false
@@ -80,7 +85,7 @@ jobs:
   # - Checks out the WordPress Test reporter repository.
   # - Submit the test results to the WordPress.org host test results.
   phpunit-tests:
-    name: PHP ${{ inputs.php }} / ${{ 'mariadb' == inputs.db-type && 'MariaDB' || 'MySQL' }} ${{ inputs.db-version }}${{ inputs.multisite && ' multisite' || '' }}${{ inputs.memcached && ' with memcached' || '' }}${{ inputs.report && ' (test reporting enabled)' || '' }} ${{ 'example.org' != inputs.tests-domain && inputs.tests-domain || '' }}
+    name: PHP ${{ inputs.php }} / ${{ 'mariadb' == inputs.db-type && 'MariaDB' || 'MySQL' }} ${{ inputs.db-version }}${{ inputs.html5lib-tests && ' (html5lib tests only)' || '' }}${{ inputs.multisite && ' multisite' || '' }}${{ inputs.memcached && ' with memcached' || '' }}${{ inputs.report && ' (test reporting enabled)' || '' }} ${{ 'example.org' != inputs.tests-domain && inputs.tests-domain || '' }}
     runs-on: ${{ inputs.os }}
     timeout-minutes: 20
 
@@ -156,19 +161,36 @@ jobs:
         run: npm run env:install
 
       - name: Run PHPUnit tests
+        if: ${{ ! inputs.html5lib-tests }}
         run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c ${{ env.PHPUNIT_CONFIG }}
 
       - name: Run AJAX tests
+        if: ${{ ! inputs.html5lib-tests }}
         run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ajax
 
       - name: Run ms-files tests as a multisite install
-        if: ${{ inputs.multisite }}
+        if: ${{ inputs.multisite && ! inputs.html5lib-tests }}
         run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ms-files
 
       - name: Run external HTTP tests
-        if: ${{ ! inputs.multisite }}
+        if: ${{ ! inputs.multisite && ! inputs.html5lib-tests }}
         run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c ${{ env.PHPUNIT_CONFIG }} --group external-http
 
+      - name: Check if the HTML API was modified
+        id: check-for-html-api-changes
+        if: ${{ inputs.html5lib-tests }}
+        uses: tj-actions/changed-files@v44
+        with:
+          files: |
+            src/wp-includes/html-api/**.php
+            tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php
+            tests/phpunit/data/html5lib-tests/**.dat
+            .github/workflows/html5lib-tests.yml
+
+      - name: Run html5lib tests
+        if: ${{ inputs.html5lib-tests && steps.check-for-html-api-changes.outputs.any_changed == 'true' }}
+        run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c ${{ env.PHPUNIT_CONFIG }} --group html-api-html5lib-tests
+
       # __fakegroup__ is excluded to force PHPUnit to ignore the <exclude> settings in phpunit.xml.dist.
       - name: Run (Xdebug) tests
         if: ${{ inputs.php != '8.3' }}
@@ -178,7 +200,7 @@ jobs:
         run: git diff --exit-code
 
       - name: Checkout the WordPress Test Reporter
-        if: ${{ github.repository == 'WordPress/wordpress-develop' && github.ref == 'refs/heads/trunk' && inputs.report }}
+        if: ${{ github.repository == 'WordPress/wordpress-develop' && github.ref == 'refs/heads/trunk' && inputs.report && ! inputs.html5lib-tests }}
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
         with:
           repository: 'WordPress/phpunit-test-runner'
@@ -186,7 +208,7 @@ jobs:
           show-progress: ${{ runner.debug == '1' && 'true' || 'false' }}
 
       - name: Submit test results to the WordPress.org host test results
-        if: ${{ github.repository == 'WordPress/wordpress-develop' && github.ref == 'refs/heads/trunk' && inputs.report }}
+        if: ${{ github.repository == 'WordPress/wordpress-develop' && github.ref == 'refs/heads/trunk' && inputs.report && ! inputs.html5lib-tests }}
         env:
           WPT_REPORT_API_KEY: "${{ secrets.WPT_REPORT_API_KEY }}"
         run: docker compose run --rm -e WPT_REPORT_API_KEY -e WPT_PREPARE_DIR=/var/www -e WPT_TEST_DIR=/var/www php php test-runner/report.php