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ARIA - why it only validates in HTML 5
The W3 ARIA recommendation specifies new HTML attributes (like ARIA-DESCRIBEDBY and ROLE) to help screen readers identify relationships between elements. These new attributes tell screen readers about relationships that can’t be derived from existing HTML semantics, and usually only obvious from the position of items on screen (e.g. a paragraph of help text next to a form field). Most people who’ve tried adding ARIA attributes to pages with HTML 4 and XHTML DOCTYPES have noticed that documents don’t validate.