Alt tags have been a big debate over that last few years. Although some companies may find that an error it is really not. Especially if the alt tag text has nothing but jibberish. As in the case of twitter profile images, they do not have a specific detailed name so we decided to omit it.
Sorta like going to w3c and running a check on html5, you’re bound to get a bunch of error and or warning about other areas of your site.
If you look at twitter and view the source for the little profile thumbs you will see they have the alt tag in their, but they do not have any text in their. SO it is basically useless markup as we see it.
No problem! As I understand it SEO works in part by taking relative information from elements on the page. In this case, as mentioned above, Twitter does not supply any alt information for the thumbnails of profiles. When they do if the content is relative we will surely add it as well to FTS 🙂