[css-fonts-4] Don't allow overriding the 0xFFFF palette entry

Author: Litherum

css-fonts-4/Overview.bs

@@ -3567,6 +3567,8 @@ The parsing rules for these descriptors are as follows:
 
 Using these rules, the non-negative integers prefixed with "color-" are valid identifiers.
 
+Integers greater than 65534 in a color-<<integer>> descriptor are invalid.
+
 These descriptors are zero-indexed, so the initial palette in the font is represented by the <code>color-0</code> descriptor.
 
 Using CSS Variables is explicitly expected to be valid in the value of this descriptor.