I just stumbled upon the use of overflow:hidden on html, which seems illogical to me:

* {
  all: unset;
  display: revert;
}
html {
  height: 100vh;
  overflow: hidden;
  background: black;
}
body {
  height: 50vh;
  font-size: 5rem;
  background: white;
  scroll-snap-type: y mandatory;
  scroll-behavior: smooth;
  overflow-y: scroll;
}

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This is about moving the scroll box from html to body but I don't understand why this example only works for overflow:hidden? The height of body is already only 50vh, so there should be no overflow on the html element, right?