diff --git a/src/testing/MD_STD_HTML_TEST.php b/src/testing/MD_STD_HTML_TEST.php
new file mode 100644
index 0000000..5b29bab
--- /dev/null
+++ b/src/testing/MD_STD_HTML_TEST.php
@@ -0,0 +1,36 @@
+
+ */
+declare(strict_types = 1);
+
+use PHPUnit\Framework\TestCase;
+
+
+/**
+ * Tests for the manifest.
+ */
+abstract class MD_STD_HTML_TEST extends TestCase {
+ /**
+ * Validates HTML strings.
+ *
+ * @param string $input Input string to validate.
+ *
+ * @return void
+ */
+ protected static function validateHtmlString(string $input):void {
+
+ self::assertNotEmpty($input);
+
+ libxml_use_internal_errors(true);
+ $xml = simplexml_load_string('' . trim(strtr($input, [' async' => ' ', ' defer' => ' '])) . '');
+ if (!empty($errors = libxml_get_errors())) {
+ throw new Exception("Invalid HTML code detected: " . var_export($errors, true) . PHP_EOL . PHP_EOL . $input);
+ }
+
+ self::assertNotEquals(null, $xml);
+
+ }
+}