Alt attribute

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: codice_1.
The attribute was introduced in HTML 2 and in HTML 4.01 was required for the codice_2 and codice_3 tags. It is optional for the codice_4 tag and the deprecated codice_5 tag.
Example.
Here is an image for which the alt attribute is "In the sky flies a red flag with a white cross whose vertical bar is shifted toward the flagpole."
The HTML for this image might be something like the following:
<img alt="In the sky flies a red flag with a white cross whose vertical bar is shifted toward the flagpole." src="http://upload.a.org/wikipedia/commons/thumb/8/83/Dannebrog.jpg/180px-Dannebrog.jpg">
A visually impaired reader using a screen reader such as Orca will hear the alt text in place of the image. A text browser such as Lynx will display the alt text instead of the image. A graphical browser typically will display only the image, and will display the alt text only if the user asks it to show the image's properties or has configured the browser not to display images, or if the browser was unable to retrieve or to decode the image.
An alternative alt attribute value would be "The Danish flag".
Common misconceptions.
The codice_6 attribute does not always have to literally describe the contents of the image. Keep in mind the purpose and context of the image and what would be useful to someone who cannot see it. The alt attribute is supposed to be an "alternative" for the image, usually stating its purpose. For example, an image of a warning sign should not have alt text “a triangle with a yellow background, black border and an exclamation mark”, but simply “Warning!”—unless, of course, the image's purpose is to show what the warning symbol actually looks like.
Internet Explorer 7 and earlier render text in alt attributes as tooltip text, which is not standards-compliant. This behavior led many web developers to misuse alt when they wished to display tooltips containing additional information about images, instead of using the codice_7 attribute that was intended for that use. As of Internet Explorer 8 alt attributes no longer render as tooltips.
The alt attribute is commonly, but incorrectly, referred to as the "alt tag".
Decorative images.
The W3C recommends that images that convey no information, but are purely decorative, be specified in CSS rather than in the HTML markup. However, it may sometimes be necessary to include a decorative image as an HTML codice_2 tag. In this case, if the image truly does not add to the content, then a blank alt attribute should be included in the form of codice_1. This makes the page navigable for users of screen readers or non-graphical browsers. If (in breach of the standard) no alt attribute has been supplied, then browsers that cannot display the image will still display something there, e.g. the URL of the image, or a fixed text string.