A Validator is an automated tool to check that coding used to create web pages is valid. It is important that coding is valid as this can impact on the accessibility of pages. Assistive technology used by disabled users such as screen readers may have problems if coding is invalid. Search engines may also have difficulty indexing pages. Most web editing packages include HTML validators or checkers, alongside spell checkers. Online validators are also available, eg W3C HTML validator and Style Sheet Validator.