diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index 50e4c940c2169..024b406876e59 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-v3.yml@trunk
+    uses: desrosj/wordpress-develop/.github/workflows/reusable-phpunit-tests-v3.yml@try/html5lib-tests
     permissions:
       contents: read
     secrets: inherit
@@ -91,6 +91,15 @@ jobs:
             multisite: false
             memcached: false
             report: true
+          # Run tests for html-api-html5lib-tests once
+          - os: ubuntu-latest
+            php: '8.3'
+            db-type: 'mysql'
+            db-version: '8.0'
+            multisite: false
+            memcached: false
+            html-api-tests: true
+
     with:
       os: ${{ matrix.os }}
       php: ${{ matrix.php }}
@@ -101,13 +110,14 @@ jobs:
       phpunit-config: ${{ matrix.multisite && 'tests/phpunit/multisite.xml' || 'phpunit.xml.dist' }}
       tests-domain: ${{ matrix.tests-domain }}
       report: ${{ matrix.report || false }}
+      html5lib-tests: ${{ matrix.html-api-tests || false }}
 
   #
   # Creates a PHPUnit test job for each PHP/MariaDB combination.
   #
   test-with-mariadb:
     name: PHP ${{ matrix.php }}
-    uses: WordPress/wordpress-develop/.github/workflows/reusable-phpunit-tests-v3.yml@trunk
+    uses: desrosj/wordpress-develop/.github/workflows/reusable-phpunit-tests-v3.yml@try/html5lib-tests
     permissions:
       contents: read
     secrets: inherit
diff --git a/.github/workflows/reusable-phpunit-tests-v3.yml b/.github/workflows/reusable-phpunit-tests-v3.yml
index 5acdbae89c338..95105028829d7 100644
--- a/.github/workflows/reusable-phpunit-tests-v3.yml
+++ b/.github/workflows/reusable-phpunit-tests-v3.yml
@@ -52,6 +52,11 @@ on:
         required: false
         type: 'string'
         default: 'example.org'
+      html5lib-tests:
+        description: 'Whether to run the html5lib tests'
+        required: false
+        type: 'boolean'
+        default: false
       report:
         description: 'Whether to report results to WordPress.org Hosting Tests'
         required: false
@@ -92,7 +97,7 @@ 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' || '' }}${{ inputs.phpunit-test-groups && format( ' ({0})', inputs.phpunit-test-groups ) || '' }}${{ 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.multisite && ' multisite' || '' }}${{ inputs.html5lib-tests && ' with html5lib tests' || '' }}${{ inputs.phpunit-test-groups && format( ' ({0})', inputs.phpunit-test-groups ) || '' }}${{ inputs.memcached && ' with memcached' || '' }}${{ inputs.report && ' (test reporting enabled)' || '' }} ${{ 'example.org' != inputs.tests-domain && inputs.tests-domain || '' }}
     runs-on: ${{ inputs.os }}
     timeout-minutes: 20
 
@@ -186,6 +191,22 @@ jobs:
         continue-on-error: ${{ inputs.allow-errors }}
         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: html-api-changes
+        if: ${{ inputs.html5lib-tests }}
+        uses: tj-actions/changed-files@cc733854b1f224978ef800d29e4709d5ee2883e4 # v45.5.5
+        with:
+          files: |
+            src/wp-includes/html-api/**.php
+            tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php
+            tests/phpunit/data/html5lib-tests/**.dat
+            .github/workflows/phpunit-tests.yml
+            .github/workflows/reusable-phpunit-tests-v3.yml
+
+      - name: Run html5lib tests
+        if: ${{ inputs.html5lib-tests && steps.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 != '8.4' && ! inputs.phpunit-test-groups }}