Content should keep its meaning however it’s presented — read aloud, restyled by the reader, or fitted to a small screen. That means the structure you can see, its headings, lists, tables and labels, has to be there in the code too, and the order things are read in has to make sense without the visual layout.
Its six success criteria cover structure and relationships, reading order, instructions that don’t rely on shape or position alone, screen orientation, and saying what each input is for.