Compound Document

A compound document is a single deliverable that seamlessly combines content in multiple formats — for example, an HTML page that embeds a Flash movie, an SVG graphic, an MathML expression, and a video player — each with its own internal document object model. Compound documents create accessibility challenges because each format has its own accessibility API and its own conventions for alternative text, roles, and keyboard handling, and assistive technology must stitch these into a single coherent experience for the user. The W3C's Compound Document Formats Working Group developed specifications for how nested formats should interoperate; today, analogous issues arise with canvas-based widgets, custom elements, and iframes that each need their own accessibility-tree contributions.