:root {
  --w: calc(100vw/1px); /* screen width  */
  --h: calc(100vh/1px); /* screen height */
  /*  The result is an integer without a unit! */
}

A version with better support:

:root {
  --w: calc(100vw/1px); /* screen width  */
  --h: calc(100vh/1px); /* screen height */
  /*  The result is an integer without a unit! */
}