vh and vw unit does not works on eink kobo

[mrbbp]

kobo aura 4.20.14622 + kepub extension

html, body {
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  background-color: black;
}
main {
  position: absolute;
  background-color: white;
  color: black;
  width: calc(100vw - 2rem);
  height: calc(100vh - 2rem);
  margin: 1rem;
}

if i use width: calc(100% - 2rem);, it works!

[mrbbp]

if i use height: 50vh;. It does not works better;