One of the purposes of the
alt attribute is to provide alternate text that can be shown when an image cannot be displayed, either because it is missing or broken, or because the user has disabled images in their (graphical) web browser.
Most graphical web browsers do display alt text in place of missing images, but there is one notable exception: Safari and most other WebKit-based browsers (iCab, Shiira, Chrome). In those browsers, all you get when an image is missing is either a small blue icon with a question mark in it or a broken image icon.
More: continued here