Added these three paragraphs to the top of the Introduction. Will this do it?

In traditional desktop graphical user interface (GUI) applications , elements of the user interface (UI) are displayed when needed and hidden when not needed based on user interactions. Accessibility application programming interfaces (APIs) are used to communicate semantic information about the user interface to assistive technology software used by people with disabilities. These APIs constitute a contract between applications and assistive technologies, such as screen readers, to enable them to access the appropriate semantics needed to produce a usable alternative to interactive applications. For example, screen reading software for blind users can determine whether a particular UI element is a menu, button, text field, list box, etc.

In traditional static Web pages, the HTML elements provided the necessary semantic information. Assistive technologies obtained the information from the Document Object Model (DOM) which did not change until a new page was loaded based on a user action. But JavaScript and CSS have enabled Web pages to behave more and more like interactive desktop GUI applications without the need to reload the page with each user interaction. Developers repurpose HTML elements into UI elements. For example, styles can be applied to a <div> element based on user interactions to make it look and behave like a popup menu. The <div> element, however, has no semantics so the screen reader doesn't understand that it is a menu.

WAI-ARIA enables rich Internet applications to have the same accessibility features as desktop GUI applications by adding metadata to markup technologies such as (X)HTML. Authors include WAI-ARIA in their markup and user agents translate the WAI-ARIA markup to the platform accessibility APIs.

The WAI-ARIA User Agent Implementation Guide is intended...