Add missing attribute decode when accessing raw class attribute HTML

Author: sirreal

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -2342,10 +2342,12 @@ private function class_name_updates_to_attributes_updates(): void {
 		}
 
 		if ( false === $existing_class && isset( $this->attributes['class'] ) ) {
-			$existing_class = substr(
-				$this->html,
-				$this->attributes['class']->value_starts_at,
-				$this->attributes['class']->value_length
+			$existing_class = WP_HTML_Decoder::decode_attribute(
+				substr(
+					$this->html,
+					$this->attributes['class']->value_starts_at,
+					$this->attributes['class']->value_length
+				)
 			);
 		}