@feonixrift XKCD puts the additional component in the "title" attribute of the image (not the "alt" one), so this is actually fine. This is common practice among many webcomics (an inherently visual art form) by the way and not specific to XKCD at all.
Unfortunately, the "alt" attribute only contains the name of the comic strip instead of an image description though, and I agree that it could be used in a much better way.
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