[css-values-4] Fix computed value and validity of mix()

Author: fantasai

css-values-4/Overview.bs

@@ -582,15 +582,24 @@ Representing Interpolated Values: the ''mix()'' notation</h3>
 	rather than the more typical comma
 	because the values themselves can contain commas.
 
-	The computed value of ''mix()'' is
-	the result of interpolating the two <<declaration-value>>s
-	to the progress given by the <<percentage>>,
-	using the interpolation rules for the property
-	in which this function is used.
-
-	A ''mix()'' notation is invalid if either of its <<declaration-value>>s
-	are invalid when interpreted as a whole value
-	for the property the function is used in,
+	If the two <<declaration-value>s in ''mix()'' are [=interpolation|interpolable=]
+	(without using ''mix()'' itself)
+	as values for the property in which it is specified,
+	the [=computed value=] of ''mix()'' is
+	the result of interpolating these two values
+	to the progress given by the <<percentage>>.
+	Otherwise,
+	the [=computed value=] of ''mix()'' is
+	the ''mix()'' [=functional notation=] itself
+	with its <<percentage>> computed
+	and both <<declaration-value>>s computed
+	as values for this property.
+
+	A ''mix()'' notation is invalid
+	if it is not the sole value of the property;
+	if either of its <<declaration-value>>s
+	are invalid when specified as the entire value
+	for the property the function is used in;
 	or if the property using it is [=not animatable=].
 
 	<div class="example">
@@ -608,18 +617,10 @@ Representing Interpolated Values: the ''mix()'' notation</h3>
 		</pre>
 	</div>
 
-	ISSUE(6700):
-	Define more precisely where mix() is allowed.
-	Is it a top-level functional notation only?
-	Or can it be nested more deeply in the grammar somehow?
-
 	Progress values below ''0%'' and above ''100%'' are valid;
 	they represent interpolation beyond the range represented
 	by the start and end values.
 
-	ISSUE(6245): Incorporate interpolation along an easing line.
-
-
 <h3 id="combining-range">
 Range Checking</h3>