Ah, of course, I didn't realize that html5test provided the useragent, it is:
Mozilla/5.0 (Nintendo Switch; WifiWebAuthApplet) AppleWebKit/601.6 (KHTML, like Gecko) NF/4.0.0.5.9 NintendoBrowser/5.1.0.13341
Which means that it's based on NetFront, the same browser that is on the PSP, 3DS, Wii U...