How to specify alternate text

Attribute definitions

alt = text [CS]
For user agents that cannot display images, forms, or applets, this attribute specifies alternate text. The language of the alternate text is specified by the lang attribute.

Several non-textual elements (IMG, AREA, APPLET, and INPUT) let authors specify alternate text to serve as content when the element cannot be rendered normally. Specifying alternate text assists users without graphic display terminals, users whose browsers don't support forms, visually impaired users, those who use speech synthesizers, those who have configured their graphical user agents not to display images, etc.

The alt attribute must be specified for the IMG and AREA elements. It is optional for the INPUT and APPLET elements.

While alternate text may be very helpful, it must be handled with care. Authors should observe the following guidelines:

* Do not specify irrelevant alternate text when including images intended to format a page, for instance, alt="red ball" would be inappropriate for an image that adds a red ball for decorating a heading or paragraph. In such cases, the alternate text should be the empty string (""). Authors are in any case advised to avoid using images to format pages; style sheets should be used instead.
* Do not specify meaningless alternate text (e.g., "dummy text"). Not only will this frustrate users, it will slow down user agents that must convert text to speech or braille output.

Implementors should consult the section on accessibility for information about how to handle cases of omitted alternate text.
Sursa http://www.w3.org/TR/REC-html40/struct/obj...s.html#adef-alt

The Alt Attribute

The alt attribute is used to define an "alternate text" for an image. The value of the alt attribute is an author-defined text:
Cod:
<img src="boat.gif" alt="Big Boat">
The "alt" attribute tells the reader what he or she is missing on a page if the browser can't load images. The browser will then display the alternate text instead of the image. It is a good practice to include the "alt" attribute for each image on a page, to improve the display and usefulness of your document for people who have text-only browsers.
Sursa: http://www.w3schools.com/html/html_images.asp

Legat de Google:
Incorrectly Assigned ALT Text

Guilty party: Amazon

Screen readers, in-car browsers and users who are connected through slow dial-up connections and turn off images rely on ALT text in place of images. There are two types of images: informational and decorative. Informational images should have an ALT description that adequately describes the image. Decorative images, on the other hand, should have ALT="" assigned to them. By assigning this null value, browsers simply ignore the image.
The majority of images on Amazon don't have any ALT text -- quite extraordinary, considering how non-time-consuming it is to assign it. Even more bizarrely, some informational images, such as the Amazon logo, have been assigned ALT="".
Sursa: http://www.sitepoint.com/article/blunders-big-player-2
Optional: http://www.gawds.org/show.php?contentid=28

Nu trebuie decat sa le citesti. Se explica foarte clar. Subiectele sint legate mai mult de artibutul ALT decat de indexarea lui de catre Google..dar sper sa iti fie de folos.
@Razvan a explicat foarte bine. Am vrut doar sa iti arat ce zic si altii.