[css-color-5] Default to oklab for color-mix(), allow color-interpolation-method to be omitted w3c#10484

Author: svgeesus

css-color-5/Overview.bs

@@ -218,7 +218,7 @@ Mixing Colors: the ''color-mix()'' Function {#color-mix}
 
 
 <pre class='prod'>
-	<dfn>color-mix()</dfn> = color-mix( <<color-interpolation-method>> , [ <<color>> && <<percentage [0,100]>>? ]#)
+	<dfn>color-mix()</dfn> = color-mix( [ <<color-interpolation-method>> ,]? [ <<color>> && <<percentage [0,100]>>? ]#)
 </pre>
 
 <wpt>
@@ -227,6 +227,13 @@ Mixing Colors: the ''color-mix()'' Function {#color-mix}
 
 </wpt>
 
+<h3 id="color-mix-space">
+	Colorspace for mixing
+</h3>
+
+If no color interpolation method is specified, assume Oklab.
+Otherwise, use the specified colorspace for mixing.
+
 <h3 id="color-mix-percent-norm">
 	Percentage Normalization
 </h3>