Actually, as other people mentioned, the real attribute that XKCD is using is "title": <img src="http://imgs.xkcd.com/comics/aspect_ratio.png" title="I'm a

Actually, as other people mentioned, the real attribute that XKCD is using is "title": <img src="http://imgs.xkcd.com/comics/aspect_ratio.png" title="I'm always disappointed when 'Anamorphic Widescreen' doesn't refer to a widescreen Animorphs movie." alt="Aspect Ratio">