diff --git a/src/Utils/HTML.php b/src/Utils/HTML.php
index d6f01360b..c2d471190 100644
--- a/src/Utils/HTML.php
+++ b/src/Utils/HTML.php
@@ -8,16 +8,22 @@ class HTML
      * Merge an associative array of attributes,
      * and make sure to merge classes and inline styles.
      */
-    public static function mergeAttributes()
+    public static function mergeAttributes(...$args): array
     {
-        $args = func_get_args();
-
-        $attributes = array_shift($args);
+        $attributes = [];
 
         foreach ($args as $moreAttributes) {
+            if (! is_array($moreAttributes) && ! is_object($moreAttributes)) {
+                continue;
+            }
+
             foreach ($moreAttributes as $key => $value) {
                 // class="foo bar"
                 if ($key === 'class') {
+                    if (! self::isStringable($value)) {
+                        continue;
+                    }
+
                     $attributes['class'] = trim(($attributes['class'] ?? '') . ' ' . $value);
 
                     continue;
@@ -25,7 +31,11 @@ public static function mergeAttributes()
 
                 // style="color: red;"
                 if ($key === 'style') {
-                    $style = rtrim($attributes['style'] ?? '', '; ') . '; ' . rtrim($value ?? '', ';') . '; ';
+                    if (! self::isStringable($value)) {
+                        continue;
+                    }
+
+                    $style = rtrim($attributes['style'] ?? '', '; ') . '; ' . rtrim($value, ';') . '; ';
                     $attributes['style'] = ltrim(trim($style), '; ');
 
                     continue;
@@ -44,24 +54,11 @@ public static function mergeAttributes()
      */
     public static function renderAttributes(array $attrs): string
     {
-        // Make boolean values a string, so they can be rendered in HTML
-        $attrs = array_map(function ($attribute) {
-            if ($attribute === true) {
-                return 'true';
-            }
-
-            if ($attribute === false) {
-                return 'false';
-            }
-
-            return $attribute;
-        }, $attrs);
-
         $attributes = [];
 
         // class="custom"
         foreach ($attrs as $name => $value) {
-            if (! is_scalar($value) || $value === '') {
+            if (! self::isStringable($value)) {
                 continue;
             }
 
@@ -69,11 +66,25 @@ public static function renderAttributes(array $attrs): string
                 $value = $value ? 'true' : 'false';
             }
 
-            $escapedValue = htmlentities((string) $value);
+            $value = (string) $value;
+
+            if ($value === '') {
+                continue;
+            }
+
+            $escapedValue = htmlentities($value);
 
             $attributes[] = " {$name}=\"{$escapedValue}\"";
         }
 
         return join($attributes);
     }
+
+    /**
+     * Whether a value can be rendered into an attribute. Excludes null.
+     */
+    private static function isStringable($value): bool
+    {
+        return is_scalar($value) || $value instanceof \Stringable;
+    }
 }
diff --git a/tests/DOMSerializer/Marks/LinkTest.php b/tests/DOMSerializer/Marks/LinkTest.php
index ba2d371e6..222b6103e 100644
--- a/tests/DOMSerializer/Marks/LinkTest.php
+++ b/tests/DOMSerializer/Marks/LinkTest.php
@@ -308,6 +308,96 @@
     expect($result)->toEqual('<a href="https://tiptap.dev">Example Link</a>');
 });
 
+test('link mark with an object class drops the attribute', function () {
+    $document = [
+        'type' => 'doc',
+        'content' => [
+            [
+                'type' => 'text',
+                'text' => 'Example Link',
+                'marks' => [
+                    [
+                        'type' => 'link',
+                        'attrs' => [
+                            'href' => 'https://tiptap.dev',
+                            'class' => ['k' => 'v'],
+                        ],
+                    ],
+                ],
+            ],
+        ],
+    ];
+
+    $result = (new Editor([
+        'extensions' => [
+            new StarterKit,
+            new Link,
+        ],
+    ]))->setContent($document)->getHTML();
+
+    expect($result)->toEqual('<a target="_blank" rel="noopener noreferrer nofollow" href="https://tiptap.dev">Example Link</a>');
+});
+
+test('link mark with an array class drops the attribute', function () {
+    $document = [
+        'type' => 'doc',
+        'content' => [
+            [
+                'type' => 'text',
+                'text' => 'Example Link',
+                'marks' => [
+                    [
+                        'type' => 'link',
+                        'attrs' => [
+                            'href' => 'https://tiptap.dev',
+                            'class' => ['a', 'b'],
+                        ],
+                    ],
+                ],
+            ],
+        ],
+    ];
+
+    $result = (new Editor([
+        'extensions' => [
+            new StarterKit,
+            new Link,
+        ],
+    ]))->setContent($document)->getHTML();
+
+    expect($result)->toEqual('<a target="_blank" rel="noopener noreferrer nofollow" href="https://tiptap.dev">Example Link</a>');
+});
+
+test('link mark with an unrenderable class keeps a class configured on the extension', function () {
+    $document = [
+        'type' => 'doc',
+        'content' => [
+            [
+                'type' => 'text',
+                'text' => 'Example Link',
+                'marks' => [
+                    [
+                        'type' => 'link',
+                        'attrs' => [
+                            'href' => 'https://tiptap.dev',
+                            'class' => ['a', 'b'],
+                        ],
+                    ],
+                ],
+            ],
+        ],
+    ];
+
+    $result = (new Editor([
+        'extensions' => [
+            new StarterKit,
+            new Link(['HTMLAttributes' => ['class' => 'tiptap']]),
+        ],
+    ]))->setContent($document)->getHTML();
+
+    expect($result)->toEqual('<a class="tiptap" href="https://tiptap.dev">Example Link</a>');
+});
+
 test('link mark can disable target', function () {
     $document = [
         'type' => 'doc',
diff --git a/tests/DOMSerializer/StyleAttributeTest.php b/tests/DOMSerializer/StyleAttributeTest.php
new file mode 100644
index 000000000..eab7ff419
--- /dev/null
+++ b/tests/DOMSerializer/StyleAttributeTest.php
@@ -0,0 +1,90 @@
+<?php
+
+namespace Tiptap\Tests\DOMSerializer;
+
+use Tiptap\Core\Node;
+use Tiptap\Editor;
+use Tiptap\Nodes\Document;
+use Tiptap\Nodes\Text;
+use Tiptap\Utils\HTML;
+
+// No bundled extension declares a literal 'style' attribute, so the style branch of
+// mergeAttributes is only reachable through a custom extension like this one.
+class StyleAttributeNode extends Node
+{
+    public static $name = 'styleAttributeNode';
+
+    public function addOptions()
+    {
+        return [
+            'HTMLAttributes' => [],
+        ];
+    }
+
+    public function addAttributes()
+    {
+        return [
+            'style' => [],
+        ];
+    }
+
+    public function parseHTML()
+    {
+        return [
+            [
+                'tag' => 'div',
+            ],
+        ];
+    }
+
+    public function renderHTML($node, $HTMLAttributes = [])
+    {
+        return ['div', HTML::mergeAttributes($this->options['HTMLAttributes'], $HTMLAttributes), 0];
+    }
+}
+
+function renderStyleAttributeNode($style, array $HTMLAttributes = [])
+{
+    $document = [
+        'type' => 'doc',
+        'content' => [
+            [
+                'type' => 'styleAttributeNode',
+                'attrs' => [
+                    'style' => $style,
+                ],
+                'content' => [
+                    [
+                        'type' => 'text',
+                        'text' => 'Example',
+                    ],
+                ],
+            ],
+        ],
+    ];
+
+    return (new Editor([
+        'extensions' => [
+            new Document,
+            new Text,
+            new StyleAttributeNode(['HTMLAttributes' => $HTMLAttributes]),
+        ],
+    ]))->setContent($document)->getHTML();
+}
+
+test('a style attribute that is a string gets rendered', function () {
+    expect(renderStyleAttributeNode('color: red'))->toEqual('<div style="color: red;">Example</div>');
+});
+
+test('a style attribute that is an array drops the attribute', function () {
+    expect(renderStyleAttributeNode(['a', 'b']))->toEqual('<div>Example</div>');
+});
+
+test('a style attribute that is an object drops the attribute', function () {
+    expect(renderStyleAttributeNode(['k' => 'v']))->toEqual('<div>Example</div>');
+});
+
+test('an unrenderable style attribute keeps a style configured on the extension', function () {
+    expect(renderStyleAttributeNode(['a', 'b'], ['style' => 'color: red']))
+        ->toEqual('<div style="color: red;">Example</div>');
+});
diff --git a/tests/Utils/HTMLTest.php b/tests/Utils/HTMLTest.php
index d5b479a98..8e54f8050 100644
--- a/tests/Utils/HTMLTest.php
+++ b/tests/Utils/HTMLTest.php
@@ -2,6 +2,22 @@
 
 use Tiptap\Utils\HTML;
 
+class StringableClass
+{
+    public function __toString(): string
+    {
+        return 'from-toString';
+    }
+}
+
+class EmptyStringableClass
+{
+    public function __toString(): string
+    {
+        return '';
+    }
+}
+
 test('classes are merged properly', function () {
     $attributes = [
         ['class' => 'a'],
@@ -58,3 +74,152 @@
 
     expect(HTML::renderAttributes($attributes))->toEqual(' target="_blank" href="https://tiptap.dev"');
 });
+
+// mergeAttributes concatenates 'class' and 'style' instead of copying them, so a value
+// that is not a string has to be dropped before it reaches the concatenation.
+test('mergeAttributes drops a class that is an object', function () {
+    $result = HTML::mergeAttributes(['class' => 'link'], ['class' => json_decode('{"k":"v"}')]);
+
+    expect($result)->toEqual(['class' => 'link']);
+});
+
+test('mergeAttributes drops a class that is an array', function () {
+    $result = HTML::mergeAttributes(['class' => 'link'], ['class' => ['a', 'b']]);
+
+    expect($result)->toEqual(['class' => 'link']);
+});
+
+test('mergeAttributes drops a class that is an object and keeps no class at all', function () {
+    $result = HTML::mergeAttributes([], ['class' => json_decode('{"k":"v"}')]);
+
+    expect($result)->toEqual([]);
+});
+
+test('mergeAttributes drops an already merged class that is an array', function () {
+    $result = HTML::mergeAttributes(['class' => ['a', 'b']], ['class' => 'external']);
+
+    expect($result)->toEqual(['class' => 'external']);
+});
+
+test('mergeAttributes drops an already merged class that is an object', function () {
+    $result = HTML::mergeAttributes(['class' => json_decode('{"k":"v"}')], ['class' => 'external']);
+
+    expect($result)->toEqual(['class' => 'external']);
+});
+
+test('mergeAttributes never renders a class when every value is unrenderable', function () {
+    $result = HTML::mergeAttributes(['class' => ['a']], ['class' => ['b']]);
+
+    expect(HTML::renderAttributes($result))->toEqual('');
+});
+
+test('mergeAttributes drops a style that is an array', function () {
+    $result = HTML::mergeAttributes(['style' => 'color: red'], ['style' => ['a', 'b']]);
+
+    expect($result)->toEqual(['style' => 'color: red;']);
+});
+
+test('mergeAttributes drops a style that is an object', function () {
+    $result = HTML::mergeAttributes(['style' => 'color: red'], ['style' => json_decode('{"k":"v"}')]);
+
+    expect($result)->toEqual(['style' => 'color: red;']);
+});
+
+test('mergeAttributes drops a style that is an object and keeps no style at all', function () {
+    $result = HTML::mergeAttributes([], ['style' => json_decode('{"k":"v"}')]);
+
+    expect($result)->toEqual([]);
+});
+
+test('mergeAttributes drops an already merged style that is an array', function () {
+    $result = HTML::mergeAttributes(['style' => ['a', 'b']], ['style' => 'font-weight: bold']);
+
+    expect($result)->toEqual(['style' => 'font-weight: bold;']);
+});
+
+test('mergeAttributes drops a style that is null instead of leaving an empty declaration', function () {
+    $result = HTML::mergeAttributes(['style' => 'color: red'], ['style' => null]);
+
+    expect($result)->toEqual(['style' => 'color: red;']);
+});
+
+// mergeAttributes has no type declarations, so callers can hand it anything.
+test('mergeAttributes accepts an object as its first argument', function () {
+    $result = HTML::mergeAttributes(json_decode('{"class":"a"}'), ['class' => 'b']);
+
+    expect($result)->toEqual(['class' => 'a b']);
+});
+
+test('mergeAttributes ignores an argument that is a string', function () {
+    $result = HTML::mergeAttributes(['class' => 'a'], 'oops');
+
+    expect($result)->toEqual(['class' => 'a']);
+});
+
+test('mergeAttributes ignores an argument that is an integer', function () {
+    $result = HTML::mergeAttributes(['class' => 'a'], 5);
+
+    expect($result)->toEqual(['class' => 'a']);
+});
+
+test('mergeAttributes ignores an argument that is null', function () {
+    $result = HTML::mergeAttributes(['class' => 'a'], null);
+
+    expect($result)->toEqual(['class' => 'a']);
+});
+
+test('mergeAttributes returns an array when called without arguments', function () {
+    expect(HTML::mergeAttributes())->toEqual([]);
+});
+
+// Guards against the fix over-reaching: every other key is copied, not concatenated,
+// and null is how callers remove an attribute again.
+test('mergeAttributes keeps a null value for keys other than class and style', function () {
+    $result = HTML::mergeAttributes(['href' => 'https://tiptap.dev'], ['target' => null]);
+
+    expect($result)->toEqual(['href' => 'https://tiptap.dev', 'target' => null]);
+});
+
+test('mergeAttributes keeps an unrenderable value for keys other than class and style', function () {
+    $result = HTML::mergeAttributes(['href' => 'https://tiptap.dev'], ['rel' => ['a', 'b']]);
+
+    expect($result)->toEqual(['href' => 'https://tiptap.dev', 'rel' => ['a', 'b']]);
+});
+
+test('mergeAttributes lets an unrenderable value overwrite an already merged one', function () {
+    $result = HTML::mergeAttributes(['rel' => 'noopener'], ['rel' => ['a', 'b']]);
+
+    expect($result)->toEqual(['rel' => ['a', 'b']]);
+});
+
+test('mergeAttributes keeps a class that is null', function () {
+    $result = HTML::mergeAttributes(['class' => 'link'], ['class' => null]);
+
+    expect($result)->toEqual(['class' => 'link']);
+});
+
+test('mergeAttributes keeps a class that is a scalar but not a string', function () {
+    $result = HTML::mergeAttributes(['class' => 'link'], ['class' => 5]);
+
+    expect($result)->toEqual(['class' => 'link 5']);
+});
+
+test('mergeAttributes keeps a class that can be cast to a string', function () {
+    $result = HTML::mergeAttributes(['class' => 'link'], ['class' => new StringableClass]);
+
+    expect($result)->toEqual(['class' => 'link from-toString']);
+});
+
+test('mergeAttributes keeps a value that can be cast to a string', function () {
+    $result = HTML::mergeAttributes([], ['title' => new StringableClass]);
+
+    expect(HTML::renderAttributes($result))->toEqual(' title="from-toString"');
+});
+
+test('renderAttributes keeps values that can be cast to a string', function () {
+    expect(HTML::renderAttributes(['title' => new StringableClass]))->toEqual(' title="from-toString"');
+});
+
+test('renderAttributes drops values that cast to an empty string', function () {
+    expect(HTML::renderAttributes(['title' => new EmptyStringableClass]))->toEqual('');
+});