.../src/Functional/CookieResourceTestTrait.php     |   2 +-
 .../EntityResource/EntityResourceTestBase.php      |   6 +-
 .../EntityResource/Node/NodeXmlAnonTest.php        |   2 +
 .../EntityResource/Node/NodeXmlBasicAuthTest.php   |  36 +++++++
 .../EntityResource/Node/NodeXmlCookieTest.php      |  31 ++++++
 .../XmlEntityNormalizationQuirksTrait.php          | 111 +++++++++++++++++++++
 .../src/Functional/XmlNormalizationQuirksTrait.php |  33 ++++++
 7 files changed, 218 insertions(+), 3 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php b/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
index 8975c3f..689fa83 100644
--- a/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
+++ b/core/modules/rest/tests/src/Functional/CookieResourceTestTrait.php
@@ -61,7 +61,7 @@ protected function initAuthentication() {
       'pass' => $this->account->passRaw,
     ];
 
-    $request_options[RequestOptions::BODY] = $this->serializer->encode($request_body, 'json');
+    $request_options[RequestOptions::BODY] = $this->serializer->encode($request_body, static::$format);
     $request_options[RequestOptions::HEADERS] = [
       'Content-Type' => static::$mimeType,
     ];
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index bb7c8eb..97a3941 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -464,8 +464,10 @@ public function testGet() {
 
     // Not only assert the normalization, also assert deserialization of the
     // response results in the expected object.
-    $unserialized = $this->serializer->deserialize((string) $response->getBody(), get_class($this->entity), static::$format);
-    $this->assertSame($unserialized->uuid(), $this->entity->uuid());
+    if (static::$format !== 'xml') {
+      $unserialized = $this->serializer->deserialize((string) $response->getBody(), get_class($this->entity), static::$format);
+      $this->assertSame($unserialized->uuid(), $this->entity->uuid());
+    }
     // Finally, assert that the expected 'Link' headers are present.
     if ($this->entity->getEntityType()->getLinkTemplates()) {
       $this->assertArrayHasKey('Link', $response->getHeaders());
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlAnonTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlAnonTest.php
index 5f8fc23..eb06390 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlAnonTest.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlAnonTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\rest\Functional\EntityResource\Node;
 
 use Drupal\Tests\rest\Functional\AnonResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
 
 /**
  * @group rest
@@ -10,6 +11,7 @@
 class NodeXmlAnonTest extends NodeResourceTestBase {
 
   use AnonResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
 
   /**
    * {@inheritdoc}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlBasicAuthTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlBasicAuthTest.php
new file mode 100644
index 0000000..93838f2
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlBasicAuthTest.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\Node;
+
+use Drupal\Tests\rest\Functional\BasicAuthResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class NodeXmlBasicAuthTest extends NodeResourceTestBase {
+
+  use BasicAuthResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['basic_auth'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'basic_auth';
+
+}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlCookieTest.php b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlCookieTest.php
new file mode 100644
index 0000000..39ae7dd
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Node/NodeXmlCookieTest.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource\Node;
+
+use Drupal\Tests\rest\Functional\CookieResourceTestTrait;
+use Drupal\Tests\rest\Functional\EntityResource\XmlEntityNormalizationQuirksTrait;
+
+/**
+ * @group rest
+ */
+class NodeXmlCookieTest extends NodeResourceTestBase {
+
+  use CookieResourceTestTrait;
+  use XmlEntityNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $format = 'xml';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $mimeType = 'text/xml; charset=UTF-8';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $auth = 'cookie';
+
+}
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/XmlEntityNormalizationQuirksTrait.php b/core/modules/rest/tests/src/Functional/EntityResource/XmlEntityNormalizationQuirksTrait.php
new file mode 100644
index 0000000..bed4f26
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/XmlEntityNormalizationQuirksTrait.php
@@ -0,0 +1,111 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource;
+
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\Plugin\Field\FieldType\BooleanItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\ChangedItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\CreatedItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
+use Drupal\Core\Field\Plugin\Field\FieldType\IntegerItem;
+use Drupal\path\Plugin\Field\FieldType\PathItem;
+use Drupal\Tests\rest\Functional\XmlNormalizationQuirksTrait;
+
+/**
+ * Trait for EntityResourceTestBase subclasses testing $format='xml'.
+ */
+trait XmlEntityNormalizationQuirksTrait {
+
+  use XmlNormalizationQuirksTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getExpectedNormalizedEntity() {
+    $default_normalization = parent::getExpectedNormalizedEntity();
+
+    $normalization = $this->applyXmlFieldDecodingQuirks($default_normalization);
+    $normalization = $this->applyXmlDecodingQuirks($normalization);
+
+    return $normalization;
+  }
+
+  /**
+   * Applies the XML entity field encoding quirks that remain after decoding.
+   *
+   * The XML encoding:
+   * - loses type data (int and bool become string)
+   *
+   * @param array $normalization
+   *   An entity normalization.
+   *
+   * @return array
+   *   The updated entity normalization.
+   *
+   * @see \Symfony\Component\Serializer\Encoder\XmlEncoder
+   */
+  protected function applyXmlFieldDecodingQuirks(array $normalization) {
+    if (!$this->entity instanceof FieldableEntityInterface) {
+      throw new \LogicException('This trait should only be used for fieldable entity types.');
+    }
+
+    foreach ($this->entity->getFields(TRUE) as $field_name => $field) {
+      // Not every field is accessible.
+      if (!isset($normalization[$field_name])) {
+        continue;
+      }
+
+      for ($i = 0; $i < count($normalization[$field_name]); $i++) {
+        switch ($field->getItemDefinition()->getClass()) {
+          case BooleanItem::class:
+            $value = &$normalization[$field_name][$i]['value'];
+            $value = $value === TRUE ? '1' : '0';
+            break;
+          case IntegerItem::class:
+            $value = &$normalization[$field_name][$i]['value'];
+            $value = (string) $value;
+            break;
+          case PathItem::class:
+            $pid = &$normalization[$field_name][$i]['pid'];
+            $pid = (string) $pid;
+            break;
+          case EntityReferenceItem::class:
+            $target_id = &$normalization[$field_name][$i]['target_id'];
+            $target_id = (string) $target_id;
+            break;
+          case ChangedItem::class:
+          case CreatedItem::class:
+            $value = &$normalization[$field_name][$i]['value'];
+            if (is_numeric($value)) {
+              $value = (string) $value;
+            }
+
+            break;
+        }
+      }
+
+      if (!empty($normalization[$field_name])) {
+        $normalization[$field_name] = $normalization[$field_name][0];
+      }
+    }
+
+    return $normalization;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testPost() {
+    // Deserialization of the XML format is not supported.
+    $this->markTestSkipped();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testPatch() {
+    // Deserialization of the XML format is not supported.
+    $this->markTestSkipped();
+  }
+
+}
diff --git a/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php b/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php
new file mode 100644
index 0000000..11c77810
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/XmlNormalizationQuirksTrait.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional;
+
+/**
+ * Trait for ResourceTestBase subclasses testing $format='xml'.
+ */
+trait XmlNormalizationQuirksTrait {
+
+  /**
+   * Applies the XML encoding quirks that remain after decoding.
+   *
+   * The XML encoding:
+   * - maps empty arrays to the empty string
+   *
+   * @param array $normalization
+   *   A normalization.
+   *
+   * @return array
+   *   The updated normalization.
+   *
+   * @see \Symfony\Component\Serializer\Encoder\XmlEncoder
+   */
+  protected function applyXmlDecodingQuirks(array $normalization) {
+    foreach ($normalization as $key => $value) {
+      if ($value === []) {
+        $normalization[$key] = '';
+      }
+    }
+    return $normalization;
+  }
+
+}