Most current browsers display the content of the alt attribute of an <img> tag only when the image itself is not displayed. To show a text value in a tooltip when you move the pointer over an image, use the title attribute instead.
To use a dedicated paragraph format for alternate text, place a paragraph containing the text in your FrameMaker document, just before the image, and assign the following properties to the paragraph format:
The Delete property prevents the alternate text from appearing as part of the HTML output. See §24.2.2 .
To provide alternate text with a marker, insert a marker of type GraphAlt in your FrameMaker document, just before the image. The content of the GraphAlt marker becomes the value of the alt attribute for the next image. See §24.2.3 and §28.2.4 .
This method is not recommended if your document includes image maps, and you also use the [GraphALT] section to assign alternate text to hotspot <area> tags. You could lose the alt content you assign to the <img> tags. See §22.10.2 .
> 22 Including graphics in HTML > 22.8 Providing alternate text for images