[css-backgrounds-4] Add note about interpolation

Author: LeaVerou

css-backgrounds-4/Overview.bs

@@ -182,7 +182,7 @@ Painting Area: the 'background-clip' property</h3>
 	    <td>the computed color
 	  <tr>
 	    <th>Animatable:
-	    <td>as <a href="https://www.w3.org/TR/css3-transitions/#animtype-color">color</a>
+	    <td>See prose
 	</table>
 
 	<table class="propdef">
@@ -221,6 +221,9 @@ Painting Area: the 'background-clip' property</h3>
 	along the direction of the side the border is applied on (i.e. split horizontally on left and right borders and vertically on top and bottom borders),
 	starting outwards.
 
+	<p>When interpolating between borders with the same number of colors, interpolation is performed individually per color band as <a href="https://www.w3.org/TR/css3-transitions/#animtype-color">color</a>. 
+		Interpolation between borders with different numbers of colors is discrete.
+
 	<p>'Border-color' is a shorthand for the
 	four 'border-*-color' properties. The four values set the top, right,
 	bottom and left border, respectively. A missing left is the same as