diff --git c/core/lib/Drupal/Component/Utility/SafeMarkup.php w/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 871cf74..d0e885d 100644
--- c/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ w/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -8,10 +8,14 @@
 namespace Drupal\Component\Utility;
 
 /**
- * Marks strings that are already escaped for XSS prevention.
+ * Manages known safe strings for rendering at the theme layer.
  *
- * @todo Add detailed documentation about how to use SafeMarkup and how it is
- *   handled during rendering and in the theme layer.
+ * As Twig autoescapes string variables in the template, it is possible for a
+ * string of markup to become double-escaped. SafeMarkup provides a store for
+ * known safe strings and methods to manage them throughout the page request.
+ * This class should be limited to interal use only. Module developers should
+ * instead use render arrays or String::checkPlain() or Xss::filter() to
+ * sanitize strings for output.
  *
  * @see sanitization
  */