If you want screen readers to describe graphical elements that illustrate important concepts in the document, you must provide the description. Figures and multimedia aren't recognized or read by a screen reader unless you add alternate text to the tag properties.
The Alt text attribute lets you create alternate text that can be read in lieu of viewing an illustration. ActualText is similar to Alt text in that it appears in lieu of an image. The ActualText attribute lets you substitute an image that is part of a word, such as using a fancy image for a drop cap.
When you export to Adobe® PDF, the Alt text and ActualText attribute values are stored in the PDF file and can be viewed in Acrobat 6.0 and later. This alternate text information can then be used when the PDF file is saved from Adobe Acrobat® software as an HTML or XML file.
You can add alt text by typing it directly into InDesign:
1. Select an image that does not have alt text.
2. With the selection tool, select the image.
3. Choose Object > Object Export Options.
4. Select the Alt Text tab in the Object Export Options dialog box.
5. Choose Custom from the Alt Text Source menu.
6. Enter the description in the text field, and click Done.
The alt text you entered is included whenever you export this InDesign file as an EPUB, HTML, or PDF document. However, it is not added to the metadata for the image file itself, so you'll need to re-enter it if you use the image in a different document. To ensure the alt text remains with the image, enter it as metadata in Adobe Bridge.
You can import alt text:
If alt text was entered for an image in Microsoft Word or Adobe Bridge, for example, you can easily assign the same text in InDesign CS6:
When you export the document, the alt text you've assigned will travel with the image.