env(viewport-segment-width/height ...) should provide RTL and BTT counting mechanisms.

[Zhang-Junzhi]

Currently, env(viewport-segment-width/height ...) can only be counted left-to-right and top-to-bottom.

For example, it's easy to target the leftmost viewport segment(or counting from left to right):

/*Put the site navigation bar in the "start"(i.e. leftmost) screen in a LTR language page*/
html:dir(ltr) .nav
{
  position: fixed;
  @media (horizontal-viewport-segments >= 2)
  {
    left: calc(env(viewport-segment-width 0 0) / 2);
    width: calc(env(viewport-segment-width 0 0) / 4);
    ...
  }
}

Targeting the topmost(or counting from top to bottom) works as easy.

But there's no as easy logically equivalent way to target the rightmost/bottommost viewport segment(or counting from bottom to top):

  /*Put the site navigation bar in the "start"(i.e. rightmost) screen in a RTL language page:
  We can only "enumerate" media queries instead of simply using "@media (horizontal-viewport-segments >= 2)":*/
html:dir(rtl) .nav
{
  position: fixed;
  @media (horizontal-viewport-segments: 2)
  {
    right: calc(env(viewport-segment-width 1 0) / 2);
    width: calc(env(viewport-segment-width 1 0) / 4);
  }
  @media (horizontal-viewport-segments: 3)
  {
    right: calc(env(viewport-segment-width 2 0) / 2);
    width: calc(env(viewport-segment-width 2 0) / 4);
  }
  @media (horizontal-viewport-segments: 4) /*I know there's basically nothing like it at the moment*/
  {
    right: calc(env(viewport-segment-width 3 0) / 2);
    width: calc(env(viewport-segment-width 3 0) / 4);
  }
}

I know there's basically really rare devices with three or even more screens at the moment, it is currently tolerable to just "enumerate" two or three media queries , but as a spec, I think CSS at least needs to theoretically support the logical symmetry.