[css-color-5] Exclude negative percentages, with @LeaVerou fix w3c#6047

Author: svgeesus

css-color-5/Overview.bs

@@ -121,33 +121,39 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 
 
 <pre class='prod'>
-	<dfn>color-mix()</dfn> = color-mix( in <<color-space>> , [ <<color>> && <<percentage>>? ]#{2})
+	<dfn>color-mix()</dfn> = color-mix( in <<color-space>> , [ <<color>> && <<percentage [0,100]>>? ]#{2})
 </pre>
 
-	The percentages are normalized as follows:
+	Percentages are required to be in the range 0% to 100%. Negative percentages are specifically disallowed. The percentages are normalized as follows:
 
 	1. Let <var>p1</var> be the first percentage and <var>p2</var> the second one.
-	2. If both percentages are omitted, they each default to 50% (an equal mix of the two colors).
+	2. If both percentages are omitted, they each default to 50%
+		(an equal mix of the two colors).
 	3. Otherwise, if <var>p2</var> is omitted, it becomes 100% - <var>p1</var>
 	4. Otherwise, if <var>p1</var> is omitted, it becomes 100% - <var>p2</var>
-	5. If the percentages sum to zero <em>do something, tbd</em>
-	6. Otherwise, if both are provided but do not add up to 100%,
-			they are scaled accordingly so that they add up to 100%.
+	5. If the percentages sum to zero, the function is invalid.
+	6. Otherwise, if both are provided and add up to greater than 100%,
+		they are scaled accordingly so that they add up to 100%.
+	7. Otherwise, if both are provided and add up to less than 100%,
+		the sum is saved as an alpha multiplier.
+		They are then scaled accordingly so that they add up to 100%.
 
 	This means that
 	<var>p1</var> becomes <var>p1</var> / (<var>p1</var> + <var>p2</var>)
 	and
 	<var>p2</var> becomes <var>p2</var> / (<var>p1</var> + <var>p2</var>).
 
-Issue: Should we handle p1 + p2 &lt; 100% differently? E.g. a mix with `transparent`?
-
 After normalizing both percentages, the result is produced via the following algorithm:
 
 1. Both colors are converted to the specified <<color-space>>.
 	If the specified color space has a smaller gamut
 	than the one in which the color to be adjusted is specified,
 	gamut mapping will occur.
 2. Colors are then <a href="../css-color-4/#interpolation">interpolated in the 		specified color space</a>.
+3. If an alpha multiplier was produced during percentage normalization,
+	the alpha component of the interpolated result is multiplied
+	by the alpha multiplier.
+
 
 The result of mixing is the color at the specified percentage along the progression
 of the second color to the first color.