You should consider how your page will render when the images are not available. Making a page readable in text-only mode can help a user assess its usefulness before images arrive. With alt text you can specify a text alternative to an image. This tests determines if you have provided an alternative text for each image in your markup.
Specifying alternative text
To specify alternative texts, you simply use the alt attribute in your img element.
<img src="images/logo.gif" alt="Company logo" />
If the alt attribute is missing, then this test will fail. However, it is acceptable to have an empty alt attribute:
<img src="images/clown.gif" alt="" />
(Note, for clarity we have not included width and height attributes in the examples above. These should be included whenever you use the img element. More info here.)
This test is based on a W3C mobileOK best practise test. See http://www.w3.org/TR/mobile-bp/#NON-TEXT_ALTERNATIVES for more details