diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..82c418f
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,53 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test (PHP ${{ matrix.php }})
+    runs-on: ubuntu-latest
+
+    strategy:
+      # Run every PHP version even if one fails so we see the full matrix.
+      fail-fast: false
+      matrix:
+        # Floor is the lowest PHP still in security support (Requires PHP: 8.2);
+        # 8.5 is the current latest. Keep in sync with the plugin's Requires PHP.
+        php: ['8.2', '8.3', '8.4', '8.5']
+
+    steps:
+      - name: Checkout plugin
+        uses: actions/checkout@v4
+
+      # The regression harness needs WordPress core's HTML API sources.
+      # Pin to a specific tag for reproducible runs. Bump this DELIBERATELY
+      # (not via automation): 7.0.0 is the latest stable release as of 2026-06
+      # (latest 6.x is 6.9.4). A shallow checkout keeps the clone small.
+      - name: Checkout WordPress source
+        uses: actions/checkout@v4
+        with:
+          repository: WordPress/wordpress-develop
+          ref: '7.0.0'
+          path: wordpress-develop
+          fetch-depth: 1
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+
+      - name: Install Composer dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run tests (regression harness + phpcs)
+        env:
+          WP_CORE_DIR: ${{ github.workspace }}/wordpress-develop/src
+        run: composer test
diff --git a/composer.json b/composer.json
index 883613d..d392ab5 100644
--- a/composer.json
+++ b/composer.json
@@ -8,5 +8,18 @@
 		"allow-plugins": {
 			"dealerdirect/phpcodesniffer-composer-installer": true
 		}
+	},
+	"scripts": {
+		"test": [
+			"@test:harness",
+			"@test:phpcs"
+		],
+		"test:harness": "php tests/html-api-integration-regression.php",
+		"test:phpcs": "phpcs"
+	},
+	"scripts-descriptions": {
+		"test": "Run the HTML API regression harness and phpcs (requires WP_CORE_DIR for the harness).",
+		"test:harness": "Run the HTML API integration regression harness (requires WP_CORE_DIR).",
+		"test:phpcs": "Run phpcs against the plugin source using phpcs.xml."
 	}
 }
diff --git a/tests/html-api-integration-regression.php b/tests/html-api-integration-regression.php
index 99dc939..88248b0 100644
--- a/tests/html-api-integration-regression.php
+++ b/tests/html-api-integration-regression.php
@@ -67,30 +67,61 @@ function do_action( $hook_name, ...$args ) {
 }
 
 if ( ! class_exists( 'WP_HTML_Processor' ) ) {
-	require "{$include_dir}/class-wp-token-map.php";
+	$required_files = array(
+		"{$include_dir}/class-wp-token-map.php",
+		"{$include_dir}/html-api/class-wp-html-text-replacement.php",
+		"{$include_dir}/html-api/class-wp-html-span.php",
+		"{$include_dir}/html-api/class-wp-html-attribute-token.php",
+		"{$include_dir}/html-api/class-wp-html-doctype-info.php",
+		"{$include_dir}/html-api/class-wp-html-unsupported-exception.php",
+		"{$include_dir}/html-api/class-wp-html-decoder.php",
+		"{$include_dir}/html-api/class-wp-html-tag-processor.php",
+		"{$include_dir}/html-api/class-wp-html-token.php",
+		"{$include_dir}/html-api/class-wp-html-stack-event.php",
+		"{$include_dir}/html-api/class-wp-html-open-elements.php",
+		"{$include_dir}/html-api/class-wp-html-active-formatting-elements.php",
+		"{$include_dir}/html-api/class-wp-html-processor-state.php",
+		"{$include_dir}/html-api/class-wp-html-processor.php",
+	);
 
-	foreach (
-		array(
-			'class-wp-html-text-replacement.php',
-			'class-wp-html-span.php',
-			'class-wp-html-attribute-token.php',
-			'class-wp-html-doctype-info.php',
-			'class-wp-html-unsupported-exception.php',
-			'class-wp-html-decoder.php',
-			'class-wp-html-tag-processor.php',
-			'class-wp-html-token.php',
-			'class-wp-html-stack-event.php',
-			'class-wp-html-open-elements.php',
-			'class-wp-html-active-formatting-elements.php',
-			'class-wp-html-processor-state.php',
-			'class-wp-html-processor.php',
-		) as $file
-	) {
-		require "{$include_dir}/html-api/{$file}";
+	// Verify every dependency exists before requiring any of them so a moved or
+	// renamed file produces a clear message instead of a fatal mid-bootstrap.
+	$missing_files = array();
+	foreach ( $required_files as $file ) {
+		if ( ! file_exists( $file ) ) {
+			$missing_files[] = $file;
+		}
+	}
+
+	if ( $missing_files ) {
+		fwrite( STDERR, "Cannot bootstrap the HTML API from WP_CORE_DIR; the following required files are missing:\n" );
+		fwrite( STDERR, '  - ' . implode( "\n  - ", $missing_files ) . "\n" );
+		fwrite( STDERR, "This WordPress version may be incompatible with the regression harness.\n" );
+		exit( 2 );
+	}
+
+	foreach ( $required_files as $file ) {
+		require $file;
 	}
 }
 
-require dirname( __DIR__ ) . '/html-api-debugger/html-api-integration.php';
+// Bail clearly if the HTML API still is not available after bootstrap.
+if ( ! class_exists( 'WP_HTML_Processor' ) ) {
+	fwrite( STDERR, "Bootstrap completed but WP_HTML_Processor is undefined; the HTML API may have changed in this WordPress version.\n" );
+	exit( 2 );
+}
+
+$integration_file = dirname( __DIR__ ) . '/html-api-debugger/html-api-integration.php';
+if ( ! file_exists( $integration_file ) ) {
+	fwrite( STDERR, "Missing plugin integration file: {$integration_file}\n" );
+	exit( 2 );
+}
+require $integration_file;
+
+if ( ! function_exists( 'HTML_API_Debugger\HTML_API_Integration\get_tree' ) ) {
+	fwrite( STDERR, "Plugin integration loaded but HTML_API_Debugger\\HTML_API_Integration\\get_tree() is undefined.\n" );
+	exit( 2 );
+}
 
 /**
  * Convert a debugger tree into stable text lines.