diff --git a/src/Parsing/ParserState.php b/src/Parsing/ParserState.php
index b60e0e06..06b083e6 100644
--- a/src/Parsing/ParserState.php
+++ b/src/Parsing/ParserState.php
@@ -9,6 +9,8 @@ class ParserState
 {
     /**
      * @var null
+     *
+     * @internal
      */
     public const EOF = null;
 
diff --git a/src/Property/AtRule.php b/src/Property/AtRule.php
index df09b5fa..64efd4de 100644
--- a/src/Property/AtRule.php
+++ b/src/Property/AtRule.php
@@ -12,6 +12,8 @@ interface AtRule extends Renderable, Commentable
      * we’re whitelisting the block rules and have anything else be treated as a set rule.
      *
      * @var string
+     *
+     * @internal
      */
     public const BLOCK_RULES = 'media/document/supports/region-style/font-feature-values';
 
@@ -19,6 +21,8 @@ interface AtRule extends Renderable, Commentable
      * … and more font-specific ones (to be used inside font-feature-values)
      *
      * @var string
+     *
+     * @internal
      */
     public const SET_RULES = 'font-face/counter-style/page/swash/styleset/annotation';
 
diff --git a/src/Property/KeyframeSelector.php b/src/Property/KeyframeSelector.php
index e44df1f9..7675252c 100644
--- a/src/Property/KeyframeSelector.php
+++ b/src/Property/KeyframeSelector.php
@@ -8,6 +8,8 @@ class KeyframeSelector extends Selector
      * regexp for specificity calculations
      *
      * @var string
+     *
+     * @internal
      */
     public const SELECTOR_VALIDATION_RX = '/
     ^(
diff --git a/src/Property/Selector.php b/src/Property/Selector.php
index 4fc734e9..0bf433d1 100644
--- a/src/Property/Selector.php
+++ b/src/Property/Selector.php
@@ -49,6 +49,8 @@ class Selector
      * regexp for specificity calculations
      *
      * @var string
+     *
+     * @internal
      */
     public const SELECTOR_VALIDATION_RX = '/
         ^(