css Beginner Questions

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It hasn't been long since I started web programming, so I'm practicing this and that Like the wallpaper of the place with the success image on the above site When I scroll down The phenomenon where the image seems to be fixed but moves little by little, I don't know how to make it background-attachment: fixed; z-index: I've tried it before. It looks similar, but it's a bit unnatural It's hard to find it even if I try to google it I would appreciate it if you could tell me roughly how to do it.