The alt attribute specifies an alternative text to be displayed in case the image fails to load.
<img src="tree.jpg" alt="Blank">
The text blank is displayed if the image tree fails to load.
The alt attribute is the Html attribute used in Html and XHtml documents to specify alternative text(alt text) that is to be rendered when the element to which it is applied cannot be rendered.
The alt attribute is used by "screen reader" software so that a person who is listening to the content of a webpage (for instance, a person who is blind) can interact with this element. Every image should have an alt attribute to be accessible, but it need not contain text. It can be an empty or null attribute: alt=""