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 cd5814f0a4ad3..910ad74a7d74a 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
@@ -1207,21 +1207,11 @@ public function class_list() {
 
 		$is_quirks = self::QUIRKS_MODE === $this->compat_mode;
 
-		$at = 0;
+		$at = strspn( $class, " \t\f\r\n" );
 		while ( $at < strlen( $class ) ) {
-			// Skip past any initial boundary characters.
-			$at += strspn( $class, " \t\f\r\n", $at );
-			if ( $at >= strlen( $class ) ) {
-				return;
-			}
-
 			// Find the byte length until the next boundary.
 			$length = strcspn( $class, " \t\f\r\n", $at );
-			if ( 0 === $length ) {
-				return;
-			}
-
-			$name = substr( $class, $at, $length );
+			$name   = substr( $class, $at, $length );
 			if ( $is_quirks ) {
 				$name = strtolower( $name );
 			}
@@ -1232,12 +1222,12 @@ public function class_list() {
 			 * Given this, it is probably faster overall to scan an array for a value rather
 			 * than to use the class name as a key and check if it's a key of $seen.
 			 */
-			if ( in_array( $name, $seen, true ) ) {
-				continue;
+			if ( ! in_array( $name, $seen, true ) ) {
+				$seen[] = $name;
+				yield $name;
 			}
 
-			$seen[] = $name;
-			yield $name;
+			$at += strspn( $class, " \t\f\r\n", $at );
 		}
 	}