The result is Zufallsshirt (“chance shirt”), a sort of Borgesian infinite library of quasi-ironic T-shirts. It generates a huge number of combinations of phrases (mostly in German, though with quite a few in English), words and clip-art. Some look like the sort of thrift-store attire hipsters wore 15 years ago, others like miscellaneous ravey wordmarks pumped out by T-shirt labels, and others are rectangles of acrostics of random words.
Some of the results are more presentable than others; one might believe that “Budapest Bicycle Flux” was a semi-obscure math-rock band whose gig the wearer happened to catch in some college-town bar back in the day, and there are situations where one might plausibly wear a T-shirt reading “I Reject Your Reality And Replace It With Cupcakes”, which, alas, cannot be said for some of the outputs, such as “your vagina is a wonderland”, or a grid of words including “Hitlerponys”, “Mörderpenis” and/or the decidedly euphemistic-sounding “wurstvuvuzela”. There are no permalinks and no way of keeping a design other than by buying it (i.e., getting it printed and shipped, which is done by a Leipzig-based mail-order T-shirt printing company). Or by saving the .png file of the image from the website. Anyway, I suspect that the creators have made a fair amount of money, and quite a few people have drawers full of odd-looking shirts from a parallel universe.
Interestingly enough, after clicking through the site for a while, a reader with a limited grasp of German may find their German comprehension improving slightly; perhaps the flood of meaningful (if nonsequiturial) sentences exercises the language pattern-matching parts of the brain in some kind of process of combinatorial fuzzing, reinforcing plausible word sequences.