diff --git a/extra/html-extra/HtmlExtension.php b/extra/html-extra/HtmlExtension.php
index 94efbe064f9..944f0c7ea50 100644
--- a/extra/html-extra/HtmlExtension.php
+++ b/extra/html-extra/HtmlExtension.php
@@ -15,8 +15,6 @@
 use Twig\Environment;
 use Twig\Error\RuntimeError;
 use Twig\Extension\AbstractExtension;
-use Twig\Extension\CoreExtension;
-use Twig\Extension\EscaperExtension;
 use Twig\Runtime\EscaperRuntime;
 use Twig\TwigFilter;
 use Twig\TwigFunction;
@@ -144,21 +142,32 @@ public static function htmlAttrMerge(...$arrays): array
                 throw new RuntimeError(sprintf('The "attr_merge" filter only works with arrays or "Traversable", got "%s" for argument %d.', \gettype($array), $argNumber + 1));
             }
 
-            $array = CoreExtension::toArray($array);
+            $array = (array) ($array);
 
-            foreach (['class', 'style', 'data', 'aria'] as $deepMergeKey) {
-                if (isset($array[$deepMergeKey])) {
-                    $value = $array[$deepMergeKey];
-                    unset($array[$deepMergeKey]);
+            foreach (['data', 'aria'] as $flatOutKey) {
+                if (!isset($array[$flatOutKey])) {
+                    continue;
+                }
+                $values = (array) $array[$flatOutKey];
+                foreach ($values as $key => $value) {
+                    $result[$flatOutKey.'-'.$key] = $value;
+                }
+                unset($array[$flatOutKey]);
+            }
 
-                    if (!is_iterable($value)) {
-                        $value = (array) $value;
-                    }
+            foreach (['class', 'style'] as $deepMergeKey) {
+                if (!isset($array[$deepMergeKey])) {
+                    continue;
+                }
 
-                    $value = CoreExtension::toArray($value);
+                $value = $array[$deepMergeKey];
+                unset($array[$deepMergeKey]);
 
-                    $result[$deepMergeKey] = array_merge($result[$deepMergeKey] ?? [], $value);
+                if (!is_iterable($value)) {
+                    $value = (array) $value;
                 }
+
+                $result[$deepMergeKey] = array_merge($result[$deepMergeKey] ?? [], $value);
             }
 
             $result = array_merge($result, $array);
diff --git a/extra/html-extra/Tests/HtmlAttrMergeTest.php b/extra/html-extra/Tests/HtmlAttrMergeTest.php
index e39c4b87b53..fb478649e68 100644
--- a/extra/html-extra/Tests/HtmlAttrMergeTest.php
+++ b/extra/html-extra/Tests/HtmlAttrMergeTest.php
@@ -19,11 +19,150 @@ public function testMerge(array $expected, array $inputs): void
 
     public function htmlAttrProvider(): \Generator
     {
-        yield 'simple test' => [
-            ['id' => 'some-id', 'class' => ['some-class']],
+        yield 'merging different attributes from two arrays' => [
+            ['id' => 'some-id', 'label' => 'some-label'],
             [
                 ['id' => 'some-id'],
-                ['class' => 'some-class'],
+                ['label' => 'some-label'],
+            ]
+        ];
+
+        yield 'merging different attributes from three arrays' => [
+            ['id' => 'some-id', 'label' => 'some-label', 'role' => 'main'],
+            [
+                ['id' => 'some-id'],
+                ['label' => 'some-label'],
+                ['role' => 'main'],
+            ]
+        ];
+
+        yield 'merging different attributes from Traversables' => [
+            ['id' => 'some-id', 'label' => 'some-label', 'role' => 'main'],
+            [
+                new \ArrayIterator(['id' => 'some-id']),
+                new \ArrayIterator(['label' => 'some-label']),
+                new \ArrayIterator(['role' => 'main']),
+            ]
+        ];
+
+        yield 'later keys override previous ones' => [
+            ['id' => 'other'],
+            [
+                ['id' => 'this'],
+                ['id' => 'that'],
+                ['id' => 'other'],
+            ]
+        ];
+
+        yield 'ignore empty strings or arrays passed as arguments' => [
+            ['some' => 'attribute'],
+            [
+                ['some' => 'attribute'],
+                [], // empty array
+                '', // empty string
+            ]
+        ];
+
+        yield 'keep "true" and "false" boolean values' => [
+            ['disabled' => true, 'enabled' => false],
+            [
+                ['disabled' => true],
+                ['enabled' => false],
+            ]
+        ];
+
+        yield 'consolidate values for the "class" key' => [
+            ['class' => ['foo', 'bar', 'baz']],
+            [
+                ['class' => ['foo']],
+                ['class' => 'bar'], // string, not array
+                ['class' => ['baz']],
+            ]
+        ];
+
+        yield 'class values can be overridden when they use names (array keys)' => [
+            ['class' => ['foo', 'bar', 'importance' => 'high']],
+            [
+                ['class' => 'foo'],
+                ['class' => ['bar', 'importance' => 'low']],
+                ['class' => ['importance' => 'high']],
+            ]
+        ];
+
+        yield 'inline style values with numerical keys are merely collected' => [
+            ['style' => ['font-weight: light', 'color: green', 'font-weight: bold']],
+            [
+                ['style' => ['font-weight: light']],
+                ['style' => ['color: green', 'font-weight: bold']],
+            ]
+        ];
+
+        yield 'inline style values can be overridden when they use names (array keys)' => [
+            ['style' => ['font-weight' => 'bold', 'color' => 'red']],
+            [
+                ['style' => ['font-weight' => 'light']],
+                ['style' => ['color' => 'green', 'font-weight' => 'bold']],
+                ['style' => ['color' => 'red']],
+            ]
+        ];
+
+        yield 'no merging happens when mixing numerically indexed inline styles with named ones' => [
+            ['style' => ['color: green', 'color' => 'red']],
+            [
+                ['style' => ['color: green']],
+                ['style' => ['color' => 'red']],
+            ]
+        ];
+
+        yield 'turning aria attributes from array to flat keys' => [
+            ['aria-role' => 'banner'],
+            [
+                ['aria' => ['role' => 'main']],
+                ['aria' => ['role' => 'banner']],
+            ]
+        ];
+
+        yield 'merging aria attributes, where the array values overrides the flat one' => [
+            ['aria-role' => 'navigation'],
+            [
+                ['aria-role' => 'main'],
+                ['aria' => ['role' => 'banner']],
+                ['aria' => ['role' => 'navigation']],
+            ]
+        ];
+
+        yield 'merging aria attributes, where the flat ones overrides the array' => [
+            ['aria-role' => 'navigation'],
+            [
+                ['aria' => ['role' => 'main']],
+                ['aria-role' => 'banner'],
+                ['aria-role' => 'navigation'],
+            ]
+        ];
+
+        yield 'turning data attributes from array to flat keys' => [
+            ['data-test' => 'bar'],
+            [
+                ['data' => ['test' => 'foo']],
+                ['data' => ['test' => 'bar']],
+            ]
+        ];
+
+        yield 'merging data attributes, where the array values overrides the flat one' => [
+            ['data-test' => 'baz'],
+            [
+                ['data-test' => 'foo'],
+                ['data' => ['test' => 'bar']],
+                ['data' => ['test' => 'baz']],
+            ]
+        ];
+
+        yield 'merging data attributes, where the flat ones overrides the array' => [
+            ['data-test' => 'baz'],
+            [
+                ['data' => ['test' => 'foo']],
+                ['data-test' => 'bar'],
+                ['data-test' => 'baz'],
             ]
         ];
     }