[css-color-5] Guarded against division by zero in color-mix() algorithm #14013

Author: svgeesus

css-color-5/Overview.bs

@@ -314,7 +314,10 @@ Otherwise, use the specified colorspace for mixing.
 				Let |combined percentage| be the sum of |a| and |b|’s percentages.
 			2. Interpolate |a| and |b|’s colors
 				as described in [[css-color-4#interpolation]],
-				with a progress percentage equal to <code>(|b|’s percentage) / |combined percentage|)</code>.
+				with a progress percentage equal to
+				<code>(|b|’s percentage) / |combined percentage|)</code>,
+				if |combined percentage| is greater than 0,
+				and 0.5 otherwise.
 				If the specified color space is a [=cylindrical polar color=] space,
 				then the <<hue-interpolation-method>> controls the
 				interpolation of hue, as described in