diff --git a/src/Core/Serialization/DOM/NormalizerFactory.php b/src/Core/Serialization/DOM/NormalizerFactory.php
index aa192834..9439bb46 100644
--- a/src/Core/Serialization/DOM/NormalizerFactory.php
+++ b/src/Core/Serialization/DOM/NormalizerFactory.php
@@ -38,15 +38,9 @@ class NormalizerFactory
 {
     public const FORMAT = Format::XML;
 
-    /**
-     * @readonly
-     */
-    private Spec $spec;
+    private readonly Spec $spec;
 
-    /**
-     * @readonly
-     */
-    private DOMDocument $document;
+    private readonly DOMDocument $document;
 
     /**
      * @throws DomainException when the spec does not support XML format
diff --git a/src/Core/Serialization/DOM/_BaseNormalizer.php b/src/Core/Serialization/DOM/_BaseNormalizer.php
index d38274c3..b59682a1 100644
--- a/src/Core/Serialization/DOM/_BaseNormalizer.php
+++ b/src/Core/Serialization/DOM/_BaseNormalizer.php
@@ -33,10 +33,7 @@
  */
 abstract class _BaseNormalizer
 {
-    /**
-     * @readonly
-     */
-    private NormalizerFactory $normalizerFactory;
+    private readonly NormalizerFactory $normalizerFactory;
 
     public function __construct(NormalizerFactory $normalizerFactory)
     {
diff --git a/src/Core/Serialization/JSON/NormalizerFactory.php b/src/Core/Serialization/JSON/NormalizerFactory.php
index f4e7abd7..fce0c17d 100644
--- a/src/Core/Serialization/JSON/NormalizerFactory.php
+++ b/src/Core/Serialization/JSON/NormalizerFactory.php
@@ -37,10 +37,7 @@ class NormalizerFactory
 {
     public const FORMAT = Format::JSON;
 
-    /**
-     * @readonly
-     */
-    private Spec $spec;
+    private readonly Spec $spec;
 
     /**
      * @throws DomainException when the spec does not support JSON format
diff --git a/src/Core/Serialization/JSON/_BaseNormalizer.php b/src/Core/Serialization/JSON/_BaseNormalizer.php
index 58afa34a..b5bea743 100644
--- a/src/Core/Serialization/JSON/_BaseNormalizer.php
+++ b/src/Core/Serialization/JSON/_BaseNormalizer.php
@@ -33,10 +33,7 @@
  */
 abstract class _BaseNormalizer
 {
-    /**
-     * @readonly
-     */
-    private NormalizerFactory $normalizerFactory;
+    private readonly NormalizerFactory $normalizerFactory;
 
     public function __construct(NormalizerFactory $normalizerFactory)
     {
diff --git a/src/Core/Serialization/JsonSerializer.php b/src/Core/Serialization/JsonSerializer.php
index 60b42e33..2ad5e188 100644
--- a/src/Core/Serialization/JsonSerializer.php
+++ b/src/Core/Serialization/JsonSerializer.php
@@ -39,11 +39,12 @@
 class JsonSerializer extends BaseSerializer
 {
     /**
-     * List of allowed options for $jsonEncodeFlags.
-     * Some flags will break the output...
+     * List of allowed options for {@see jsonEncodeFlags}.
      *
      * Bitmask consisting of JSON_*.
      *
+     * Some JSON flags could break the output, so they are not whitelisted.
+     *
      * @see https://www.php.net/manual/en/json.constants.php
      */
     private const JsonEncodeFlagsAllowedOptions = 0
@@ -56,6 +57,20 @@ class JsonSerializer extends BaseSerializer
         | \JSON_UNESCAPED_UNICODE
     ;
 
+    /**
+     * Defaults of {@see $jsonEncodeFlags}.
+     *
+     * Bitmask consisting of JSON_*.
+     *
+     * These defaults are required to have valid output in the end.
+     *
+     * @see https://www.php.net/manual/en/json.constants.php
+     */
+    private const JsonEncodeFlagsDefaults = 0
+        | \JSON_THROW_ON_ERROR // prevent unexpected data
+        | \JSON_PRESERVE_ZERO_FRACTION // float/double not converted to int
+    ;
+
     /**
      * List of mandatory options for $jsonEncodeFlags.
      *
@@ -67,8 +82,7 @@ class JsonSerializer extends BaseSerializer
         | \JSON_UNESCAPED_SLASHES // urls become shorter
     ;
 
-    /** @readonly */
-    private JSON\NormalizerFactory $normalizerFactory;
+    private readonly JSON\NormalizerFactory $normalizerFactory;
 
     /**
      * Flags for {@see \json_encode()}.
@@ -76,14 +90,8 @@ class JsonSerializer extends BaseSerializer
      * Bitmask consisting of JSON_*.
      *
      * @see https://www.php.net/manual/en/json.constants.php
-     *
-     * @readonly
      */
-    private int $jsonEncodeFlags = 0
-        // These defaults are required to have valid output.
-        | \JSON_THROW_ON_ERROR // prevent unexpected data
-        | \JSON_PRESERVE_ZERO_FRACTION // float/double not converted to int
-    ;
+    private readonly int $jsonEncodeFlags;
 
     /**
      * @param int $jsonEncodeFlags Bitmask consisting of JSON_*. see {@see JsonEncodeFlagsAllowedOptions}
@@ -93,7 +101,8 @@ public function __construct(
         int $jsonEncodeFlags = self::JsonEncodeFlagsDefaultOptions
     ) {
         $this->normalizerFactory = $normalizerFactory;
-        $this->jsonEncodeFlags |= $jsonEncodeFlags & self::JsonEncodeFlagsAllowedOptions;
+        $this->jsonEncodeFlags = self::JsonEncodeFlagsDefaults
+            | ($jsonEncodeFlags & self::JsonEncodeFlagsAllowedOptions);
     }
 
     protected function realNormalize(Bom $bom): array
diff --git a/src/Core/Serialization/XmlSerializer.php b/src/Core/Serialization/XmlSerializer.php
index a578f6cf..78b36a13 100644
--- a/src/Core/Serialization/XmlSerializer.php
+++ b/src/Core/Serialization/XmlSerializer.php
@@ -38,13 +38,10 @@
  */
 class XmlSerializer extends BaseSerializer
 {
-    /** @readonly */
-    private DOM\NormalizerFactory $normalizerFactory;
+    private readonly DOM\NormalizerFactory $normalizerFactory;
 
-    /** @readonly */
-    private string $xmlVersion;
-    /** @readonly */
-    private string $xmlEncoding;
+    private readonly string $xmlVersion;
+    private readonly string $xmlEncoding;
 
     public function __construct(
         DOM\NormalizerFactory $normalizerFactory,
diff --git a/src/Core/Validation/BaseValidator.php b/src/Core/Validation/BaseValidator.php
index a9945aff..4df2f570 100644
--- a/src/Core/Validation/BaseValidator.php
+++ b/src/Core/Validation/BaseValidator.php
@@ -30,10 +30,7 @@
  */
 abstract class BaseValidator implements Validator
 {
-    /**
-     * @readonly
-     */
-    private Spec $spec;
+    private readonly Spec $spec;
 
     public function __construct(Spec $spec)
     {
diff --git a/src/Core/Validation/ValidationError.php b/src/Core/Validation/ValidationError.php
index b8093fe6..4de9efaa 100644
--- a/src/Core/Validation/ValidationError.php
+++ b/src/Core/Validation/ValidationError.php
@@ -30,10 +30,7 @@
  */
 class ValidationError
 {
-    /**
-     * @readonly
-     */
-    private string $message;
+    private readonly string $message;
 
     /**
      * keep for internal debug purposes.