Carrhotus xanthogramma: the closest I've ever come to enjoying a spider video.

And a couple of (lightly edited) notes from the uploader, sigma1920:

An adult male of the jumping spider (Carrhotus xanthogramma, family Salticidae) showing combat displays against his own reflection on the mirror again and again. Mid-May 2009 in Japan. Soundtrack (audioswapped): "Into The Mirror" by Spencer Brewer.

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This species (Carrhotus xanthogramma) is one of the commonest jumping spider in Japan. The mature males and females have quite different colourations (sexual dimorphism) and you can tell them apart easily. Unlike the general rule-of-thumb, the palpi of adult male C. xanthogramma are not so bulbous.