The /deep/ combinator was a part of Shadow DOM v0, which has been deprecated and removed. Starting in M63, the /deep/ combinator was treated as a no-op, equivalent to a space " " combinator. As the code for all of Shadow DOM v0 was removed completely in M89, /deep/ will now throw exceptions in some JS operations, such as querySelectorAll. Simply replace it with " " to get pre-M89 behavior back.