The alt property sets or returns the value of the alt attribute of an image.
The required alt attribute specifies an alternate text for an image, if the image for some reason cannot be displayed (because of slow connection, an error in the src attribute, or if the user uses a screen reader).
Note: Internet Explorer displays the value of the alt attribute as a tooltip when mousing over the img element. This is NOT the correct behavior, according to the HTML specification. All other browsers are following the specification, and will only display the alt text if the image cannot be displayed.