Markup is a language or code to tell other technologies how to interact with content. For example, HTML is a markup language that tells technologies such as a web browser or screen reader technology how to present text and controls to both sighted and blind users. If the proper markup is missing, there is no guarantee about how various technologies will interact with it.
Relevant issues from the database include: Lack of Separation Between Individual Controls, Content Is Not Keyboard Accessible, and Element Does Not Indicate Its Current State. These are three issues that come out of markup failing to give screen reading software the necessary information.