diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index 7ca5191a0f162..526206f19eb2d 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -1207,11 +1207,12 @@ public function class_list() {
 
 		$is_quirks = self::QUIRKS_MODE === $this->compat_mode;
 
-		$at = 0;
-		while ( $at < strlen( $class ) ) {
+		$at           = 0;
+		$class_length = strlen( $class );
+		while ( $at < $class_length ) {
 			// Skip past any initial boundary characters.
 			$at += strspn( $class, " \t\f\r\n", $at );
-			if ( $at >= strlen( $class ) ) {
+			if ( $at >= $class_length ) {
 				return;
 			}