How can I add alt for anibox shortcode?
If your WordPress theme allows using anibox shortcodes for animated hover effects on images, for example, as presented on this demo site https://demo.pixelemu.com/pe-school-wcag/animated-gallery/ , then it's enough to add ALT parameter in the shortcode, for example:
[anibox effect="sadie" background="http://placehold.it/480x360" alt="Lorem ipsum" fontcolor="#000000" width="480" height="360" link="#" title="Example Title" titletag="h2" subtitle="Lorem ipsum dolor sit amet."]
Here you may check the all available parameters for anibox shortcode: https://demo.pixelemu.com/pe-school-wcag/shortcodes/