When a you try to access a page that doesn't exist on a website, servers send back a 404 error. Optionally, they can send back a page explaining the situation, and many sites use this space to be creative. In fact, the Krystal Archive 404 page does just that!
Anyway, Reddit user NicoDarunia discovered that the official European Nintendo website features a Peppy Hare-themed 404 page, complete with a button that makes the website do a barrel roll.
Try it yourself: Go to the official Krystal Smash Bros Announcement page.