diff --git a/core/lib/Drupal/Core/Entity/ContentEntityType.php b/core/lib/Drupal/Core/Entity/ContentEntityType.php
index 0e26c3bb51..c4033b5dbb 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityType.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityType.php
@@ -17,6 +17,11 @@ class ContentEntityType extends EntityType implements ContentEntityTypeInterface
   /**
    * {@inheritdoc}
    */
+  protected $supports_validation = TRUE;
+
+  /**
+   * {@inheritdoc}
+   */
   public function __construct($definition) {
     parent::__construct($definition);
     $this->handlers += [
diff --git a/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
index b39fb3fa46..64127a8290 100644
--- a/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -278,6 +278,13 @@ class EntityType extends PluginDefinition implements EntityTypeInterface {
   protected $additional = [];
 
   /**
+   * Supports validation of the entity type, useful for REST api support.
+   *
+   * @var bool
+   */
+  protected $supports_validation = FALSE;
+
+  /**
    * Constructs a new EntityType.
    *
    * @param array $definition
diff --git a/core/modules/config/tests/config_test/src/Entity/ConfigTest.php b/core/modules/config/tests/config_test/src/Entity/ConfigTest.php
index c432c5bf2c..4f84ecfe1f 100644
--- a/core/modules/config/tests/config_test/src/Entity/ConfigTest.php
+++ b/core/modules/config/tests/config_test/src/Entity/ConfigTest.php
@@ -22,7 +22,7 @@
  *     },
  *     "access" = "Drupal\config_test\ConfigTestAccessControlHandler"
  *   },
- *   rest_resource = TRUE,
+ *   supports_validation = TRUE,
  *   config_prefix = "dynamic",
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/rest/src/Plugin/rest/resource/ConfigEntityResource.php b/core/modules/rest/src/Plugin/rest/resource/ConfigEntityResource.php
index 7e16e6e3ac..9ca05f01f8 100644
--- a/core/modules/rest/src/Plugin/rest/resource/ConfigEntityResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/ConfigEntityResource.php
@@ -15,9 +15,7 @@ class ConfigEntityResource extends EntityResource {
    * {@inheritdoc}
    */
   protected function validate(EntityInterface $entity) {
-    // Add a custom config entity resource class in order to provide
     /** @var \Drupal\Core\Config\Entity\ConfigEntityInterface $entity */
-
     $config = \Drupal::configFactory()->getEditable($entity->getConfigDependencyName());
     $config->setData($entity->toArray());
 
@@ -25,15 +23,7 @@ protected function validate(EntityInterface $entity) {
 
     $violations = $typed_config->validate();
 
-    if ($violations->count() > 0) {
-      $message = "Unprocessable Entity: validation failed.\n";
-      foreach ($violations as $violation) {
-        // We strip every HTML from the error message to have a nicer to read
-        // message on REST responses.
-        $message .= $violation->getPropertyPath() . ': ' . PlainTextOutput::renderFromHtml($violation->getMessage()) . "\n";
-      }
-      throw new UnprocessableEntityHttpException($message);
-    }
+    $this->processViolations($violations);
   }
 
 }
diff --git a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
index cc7fe4a1b3..43e560e76e 100644
--- a/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
+++ b/core/modules/rest/src/Plugin/rest/resource/EntityResource.php
@@ -394,7 +394,7 @@ protected function getBaseRoute($canonical_path, $method) {
    */
   public function availableMethods() {
     $methods = parent::availableMethods();
-    if ($this->isConfigEntityResource() && empty($this->entityType->get('rest_resource'))) {
+    if ($this->entityType->get('supports_validation')) {
       // Currently only GET is supported for Config Entities.
       // @todo Remove when supported https://www.drupal.org/node/2300677
       $unsupported_methods = ['POST', 'PUT', 'DELETE', 'PATCH'];
diff --git a/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php b/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php
index 09b4b64bae..25a9281543 100644
--- a/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php
+++ b/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php
@@ -33,6 +33,18 @@ protected function validate(EntityInterface $entity) {
     // changes.
     $violations->filterByFieldAccess();
 
+    $this->processViolations($violations);
+  }
+
+  /**
+   * Processes violations and creates a helpful exception message.
+   *
+   * @param \Drupal\Core\Entity\EntityConstraintViolationListInterface $violations
+   *
+   * @throws \Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException
+   *   Throws a HTTP exception when the validation fails.
+   */
+  protected function processViolations($violations) {
     if ($violations->count() > 0) {
       $message = "Unprocessable Entity: validation failed.\n";
       foreach ($violations as $violation) {
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
index 1ec31a5ea3..60ebb53e04 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/ConfigTest/ConfigTestResourceTestBase.php
@@ -22,12 +22,12 @@
    */
   protected $entity;
 
-  protected $counter = 0;
-
   /**
-   * {@inheritdoc}
+   * Countered used internally to have deterministic ids.
+   *
+   * @var int
    */
-  protected $configEntityWithModifyingTest = TRUE;
+  protected $counter = 0;
 
   /**
    * {@inheritdoc}
@@ -38,11 +38,7 @@ protected function setUpAuthorization($method) {
         $this->grantPermissionsToTestedRole(['view config_test']);
         break;
       case 'POST':
-        $this->grantPermissionsToTestedRole(['administer config_test']);
-        break;
       case 'PATCH':
-        $this->grantPermissionsToTestedRole(['administer config_test']);
-        break;
       case 'DELETE':
         $this->grantPermissionsToTestedRole(['administer config_test']);
         break;
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index ebad9546c1..730e6550d1 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -131,13 +131,6 @@
   public static $modules = ['rest_test', 'text'];
 
   /**
-   * Flag to indicate a test which can modify config entities.
-   *
-   * @var bool
-   */
-  protected $configEntityWithModifyingTest = FALSE;
-
-  /**
    * Provides an entity resource.
    */
   protected function provisionEntityResource() {
@@ -592,8 +585,8 @@ protected static function castToString(array $normalization) {
    */
   public function testPost() {
     // @todo Remove this in https://www.drupal.org/node/2300677.
-    if ($this->entity instanceof ConfigEntityInterface && !$this->configEntityWithModifyingTest) {
-      $this->assertTrue(TRUE, 'POSTing config entities is not yet supported.');
+    if (!$this->entity->getEntityType()->get('supports_validation')) {
+      $this->assertTrue(TRUE, "This entity type doesn't support posting.");
       return;
     }
 
@@ -605,6 +598,8 @@ public function testPost() {
     $parseable_valid_request_body   = $this->serializer->encode($this->getNormalizedPostEntity(), static::$format);
     $parseable_valid_request_body_2 = $this->serializer->encode($this->getNormalizedPostEntity(), static::$format);
     $parseable_invalid_request_body   = $this->serializer->encode($this->makeNormalizationInvalid($this->getNormalizedPostEntity()), static::$format);
+    // The normalized structure looks different for fieldable and non fieldable
+    // entities.
     if ($this->entity instanceof FieldableEntityInterface) {
       $parseable_invalid_request_body_2 = $this->serializer->encode($this->getNormalizedPostEntity() + ['uuid' => [$this->randomMachineName(129)]], static::$format);
     }
@@ -696,13 +691,11 @@ public function testPost() {
     $this->setUpAuthorization('POST');
 
 
-    if ($this->entity instanceof FieldableEntityInterface) {
-      // DX: 422 when invalid entity: multiple values sent for single-value field.
-      $response = $this->request('POST', $url, $request_options);
-      $label_field = $this->entity->getEntityType() ->hasKey('label') ? $this->entity->getEntityType() ->getKey('label') : static::$labelFieldName;
-      $label_field_capitalized = $this->entity->getFieldDefinition($label_field) ->getLabel();
-      $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\n$label_field: $label_field_capitalized: this field cannot hold more than 1 values.\n", $response);
-    }
+    // DX: 422 when invalid entity: multiple values sent for single-value field.
+    $response = $this->request('POST', $url, $request_options);
+    $label_field = $this->entity->getEntityType() ->hasKey('label') ? $this->entity->getEntityType() ->getKey('label') : static::$labelFieldName;
+    $label_field_capitalized = $this->entity->getFieldDefinition($label_field) ->getLabel();
+    $this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\n$label_field: $label_field_capitalized: this field cannot hold more than 1 values.\n", $response);
 
     $request_options[RequestOptions::BODY] = $parseable_invalid_request_body_2;
 
@@ -795,9 +788,8 @@ public function testPost() {
    * Tests a PATCH request for an entity, plus edge cases to ensure good DX.
    */
   public function testPatch() {
-    // @todo Remove this in https://www.drupal.org/node/2300677.
-    if ($this->entity instanceof ConfigEntityInterface && !$this->configEntityWithModifyingTest) {
-      $this->assertTrue(TRUE, 'PATCHing config entities is not yet supported.');
+    if (!$this->entity->getEntityType()->get('supports_validation')) {
+      $this->assertTrue(TRUE, "This entity type doesn't support patching.");
       return;
     }
 
@@ -996,9 +988,8 @@ public function testPatch() {
    * Tests a DELETE request for an entity, plus edge cases to ensure good DX.
    */
   public function testDelete() {
-    // @todo Remove this in https://www.drupal.org/node/2300677.
-    if ($this->entity instanceof ConfigEntityInterface && !$this->configEntityWithModifyingTest) {
-      $this->assertTrue(TRUE, 'DELETEing config entities is not yet supported.');
+    if (!$this->entity->getEntityType()->get('supports_validation')) {
+      $this->assertTrue(TRUE, "This entity type doesn't support deletion.");
       return;
     }
 
@@ -1171,7 +1162,16 @@ protected function getPostUrl() {
   protected function makeNormalizationInvalid(array $normalization) {
     // Add a second label to this entity to make it invalid.
     $label_field = $this->entity->getEntityType()->hasKey('label') ? $this->entity->getEntityType()->getKey('label') : static::$labelFieldName;
-    $normalization[$label_field][1]['value'] = 'Second Title';
+
+    if ($this->entity instanceof FieldableEntityInterface) {
+      $normalization[$label_field][1]['value'] = 'Second Title';
+    }
+    else {
+      $normalization[$label_field] = [
+        $normalization[$label_field],
+        'Second title',
+      ];
+    }
 
     return $normalization;
   }