Markup is considered well-formed when which of the following criteria are met?

Markup is considered well-formed when elements are properly nested. This means that every opening tag has a corresponding closing tag, and the structure of the document follows a hierarchy that reflects the relationships among the elements. Proper nesting ensures that the markup is easily understood by both browsers and assistive technologies, which is crucial for rendering content correctly and for making it accessible to all users.

While checking spelling, validating forms, and ensuring unique id attributes are important practices for overall code quality and accessibility, they do not directly relate to the "well-formed" status of markup. These aspects enhance user experience and can contribute to accessibility but do not determine if the markup structure itself adheres to the standards of being well-formed. Proper nesting, on the other hand, is a fundamental requirement in markup languages like HTML and XML.