[css-values-5] Specify that zero/small steps get ignored. w3c#11634

Author: tabatkins

css-values-5/Overview.bs

@@ -2120,17 +2120,23 @@ Argument Ranges</h4>
 	In ''random(A, B, by C)'',
 	if A or B is infinite,
 	the result is NaN.
-	If C is infinite,
-	the result is A.
 
 	If A and B are finite,
 	but the distance between them (''B - A'')
 	is large enough to become infinite in the user agent,
 	the result is NaN.
 
-	(If C is zero or negative,
-	the result is A,
-	but that falls out of the standard definition.)
+	If C is negative and non-zero,
+	the result is NaN.
+	If C is infinite,
+	the result is A.
+
+	If C is zero,
+	or positive but close enough to zero
+	that the range for the step multiplier
+	(the <code>N</code> mentioned in the argument's definition)
+	would be infinite in the user agent,
+	the step must be <em>ignored</em>.
 
 	Note: As usual for [=math functions=],
 	if any argument calculation is NaN,