Dogs wag their tails when they are happy. More stories about dogs follow. any actions that change the display must change the equivalent @@Is this computable in a practical time (cf. NP complete) .  Computer science help needed here.  Of course, as in other parts of document, the fact that the equivalent changes is no guarantee that equivalent is correct than it is guaranteed that "alt" text for an image is correct. Cats sleep a lot. Here is more information about cats... technologies have been reviewed for accessibility issues throughout the design phase and have accessibility built-in. Many non-W3C formats require specific applications (often called plug-ins) that are inaccessible or do not work well with assistive technologies. Using W3C technologies will tend to make pages more accessible to more people using a wider variety of hardware and software