[Color 4] Be stricter about slash-separated strings (#3862)

Co-authored-by: Carlos (Goodwine) <2022649+Goodwine@users.noreply.github.com>

accepted/color-4-new-spaces.changes.md

@@ -1,3 +1,8 @@
+## Draft 1.19
+
+* Be stricter about which colors are allowed with slash-separated strings at the
+  end.
+
 ## Draft 1.18
 
 * Treat missing channels as distinct from 0 for `==`-equality for colors.

accepted/color-4-new-spaces.md

@@ -1138,29 +1138,25 @@ The procedure is:
 
   * If the last element of `input` is an unquoted string that contains `/`:
 
+    > This solves for a legacy handling of `/` in Sass that would produce an
+    > unquoted string when the alpha value is a CSS function such as `var()`,
+    > when either value is `none`, or when using relative color syntax.
+
     * Let `split-last` be the result calling `string.split()` with the last
       element of `input` as the string to split, and `/` as the separator.
 
-    * If `split-last` has two items, and one or both items are an unquoted
-      string that's case-insensitively equal to 'none':
-
-      > Special handling for `none/none`, `none/<number>`, and `<number>/none`.
-
-      * If either item in `split-last` can be coerced to a number, replace
-        the current value of the item with the resulting number value.
-
-      * If any item in `split-last` is not a number or an unquoted string
-        that's case-insensitively equal to 'none', return an unquoted string
-        with the value of `input`.
+    * If `split-last` doesn't have exactly two items, return an unquoted string
+      with the value of `input`.
 
-      * Otherwise, let `alpha` be the second element in `split-last`, and
-        append the first element of `split-last` to `components`.
+      > This ensures that `rgb(1 2 calc(var(--a) / var(--b)) / var(--c))` is
+      > handled correctly after the final expresssion is fully converted to a
+      > string due to legacy `/` behavior.
 
-    * Otherwise, return an unquoted string with the value of `input`.
+    * If either item in `split-last` can be coerced to a number, replace
+      the current value of the item with the resulting number value.
 
-      > This solves for a legacy handling of `/` in Sass that would produce an
-      > unquoted string when the alpha value is a CSS function such as `var()`
-      > or when either value is `none`.
+    * Let `alpha` be the second element in `split-last`, and append the first
+      element of `split-last` to `components`.
 
   * Otherwise, if the last element of `input` has preserved its status as two
     slash-separated numbers: