TL;DR

@property --_w {
  syntax: '<length>';
  inherits: true;
  initial-value: 100vw; 
}
@property --_h {
  syntax: '<length>';
  inherits: true;
  initial-value: 100vh; 
}
:root {
  --w: tan(atan2(var(--_w),1px)); /* screen width */
  --h: tan(atan2(var(--_h),1px)); /* screen height*/
  /* The result is an integer without unit  */
}