A person viewing through a screen reader should know the message that the image is conveying. If the images are only decorative, then use of an empty tag will suffice. But when the image contains relevant information, then a text equivalent in the alt tag should be provided.

If a video file or an animated flash file is created, a synchronized audio file should also be created to match audio with captions and the flow of the animation.

Even when a user switches off style sheets, the content should be readable by the user. Using CSS positioning attributes is a bad idea because unexpected results can appear if a user switches off the style sheets.

See example of course with style sheets switched off. The content is still clearly readable and the only difference is the lack of formatting and some background images, which are used only for a decorative purpose.

If it is not possible to make a page accessible due to constraints, then provide an accessible text only page of the same. This is illustrated in the diagram