Render backed enums using their backing value in the html_attr function

CHANGELOG

@@ -1,5 +1,6 @@
 # 3.28.0 (2026-XX-XX)
 
+ * Render backed enums using their backing value in the `html_attr` function
  * Fix empty Markup values being treated as truthy in and, or, xor, not, ternary, and elvis expressions
  * Fix a PHP 8.5 `chr()` deprecation when decoding an octal string escape sequence larger than `\377` (such as `"\777"`)
  * Mark `Twig\Markup` as `@final`; it will be final in Twig 4.0

extra/html-extra/HtmlExtension.php

@@ -200,6 +200,10 @@ final class HtmlExtension extends AbstractExtension
         $runtime = $env->getRuntime(EscaperRuntime::class);
 
         foreach ($attr as $name => $value) {
+            if ($value instanceof \BackedEnum) {
+                $value = $value->value;
+            }
+
             if (str_starts_with($name, 'aria-')) {
                 // For aria-*, convert booleans to "true" and "false" strings
                 if (true === $value) {

extra/html-extra/Tests/HtmlAttrTest.php

@@ -247,6 +247,42 @@ class HtmlAttrTest extends TestCase
                 ['value' => new StringableStub('stringable-object')],
             ],
         ];
+
+        // Backed enums are rendered using their backing value
+        yield 'string-backed enum renders its value' => [
+            'class="card"',
+            [
+                ['class' => StringBackedStub::CARD],
+            ],
+        ];
+
+        yield 'int-backed enum renders its value' => [
+            'tabindex="10"',
+            [
+                ['tabindex' => IntBackedStub::HIGH],
+            ],
+        ];
+
+        yield 'string-backed enum in data-* attribute renders its value without JSON encoding' => [
+            'data-view="card"',
+            [
+                ['data-view' => StringBackedStub::CARD],
+            ],
+        ];
+
+        yield 'int-backed enum in data-* attribute renders its value' => [
+            'data-level="10"',
+            [
+                ['data-level' => IntBackedStub::HIGH],
+            ],
+        ];
+
+        yield 'backed enum in aria-* attribute renders its value' => [
+            'aria-label="card"',
+            [
+                ['aria-label' => StringBackedStub::CARD],
+            ],
+        ];
     }
 
     public function testIterableObjectCastedToArray()
@@ -317,3 +353,15 @@ class AttributeValueStub implements AttributeValueInterface
         return $this->value;
     }
 }
+
+enum StringBackedStub: string
+{
+    case CARD = 'card';
+    case TABLE = 'table';
+}
+
+enum IntBackedStub: int
+{
+    case LOW = 1;
+    case HIGH = 10;
+}