Wolfgang Ziegler
--- a/src/CustomElementNormalizer.php
+++ b/src/CustomElementNormalizer.php
+    // Special handling for "renderless-container" elements. Only slots are
+    // output, the element itself is not rendered.
+    if ($element->getTag() == 'renderless-container') {
+      // Merge all slots into a single array.
+      return call_user_func_array('array_merge', array_values($normalized_slots));
+    }
+    else {
+      $result = array_merge($result, $normalized_slots);
+    }
+
+    // Special handling for "div" and "span" elements in JSON output.
+    // Those elements are used to wrap slot-content and should not be output as
+    // separate elements in the JSON structure.
+    // @todo Consider using renderless-container instead of div/span in
+    // in slot-helper-methods of CustomElement class.
+    if ($result['element'] == 'div' || $result['element'] == 'span') {
--- a/tests/src/Traits/TestDebugHelperTrait.php
+++ b/tests/src/Traits/TestDebugHelperTrait.php
+            $result[$key] = $normalizer->normalize($value);
+          }
+          else {
+            $result[$key] = $value;
+          }
        }
-        catch (\Exception) {
-          // Fallback to basic conversion if serialization fails.
-          return $this->convertToSerializable($data);
-        }
-      };
-    }
-    else {
-      // Fallback for when the service isn't available.
-      $serialize = function ($data) {
-        return $this->convertToSerializable($data);
--- a/tests/src/Kernel/RenderlessContainerTest.php 0 → 100644
+++ b/tests/src/Kernel/RenderlessContainerTest.php 0 → 100644
+
+  /**
+   * Tests renderless-container with nested elements.
+   */
+  public function testRenderlessContainerNested(): void {
+    // Create a more complex structure with nested elements.
+    $container = CustomElement::create('renderless-container');
+
+    // Create a complex child with its own slots.
+    $complexChild = CustomElement::create('article')
+      ->setAttribute('class', 'article-wrapper');
+
+    $header = CustomElement::create('header')
+      ->setSlot('default', 'Article Header');
+
+    $content = CustomElement::create('div')
--- a/custom_elements.module
+++ b/custom_elements.module
+      'base hook' => 'custom_element',
+    ],
  ];
 }

+/**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function custom_elements_theme_suggestions_custom_element(array $variables) {
+  $suggestions = [];
+  /** @var \Drupal\custom_elements\CustomElement $custom_element */
+  $custom_element = $variables['custom_element'];
+
+  // Add suggestion based on tag name.
+  $tag = str_replace('-', '_', $custom_element->getTag());
+  $suggestions[] = 'custom_element__' . $tag;