[css-shapes-1] Define that all the rectangle functions compute to inset().

Author: tabatkins

css-shapes-1/Overview.bs

@@ -590,14 +590,21 @@ Computed Values of Basic Shapes</h3>
 	<ul>
 		<li>
 			Omitted values are included and compute to their defaults.
-		</li>
+
 		<li>
 			A <<position>> value in ''circle()'' or ''ellipse()'' is computed as a pair of offsets (horizontal then vertical) from the top left origin, each given as a <<length-percentage>>.
-		</li>
+
 		<li>
 			A <<'border-radius'>> value in a <<basic-shape-rect>> function
 			is computed as an expanded list of all eight <<length-percentage>> values.
-		</li>
+
+		<li>
+			All <<basic-shape-rect>> functions compute to the equivalent ''inset()'' function.
+
+			Note: Given ''rect(t r b l)'',
+			the equivalent function is ''inset(t calc(100% - r) calc(100% - b) l)''.
+			Given ''xywh(x y w h)'',
+			the equivalent function is ''inset(y calc(x + w) calc(y + h) x)''.
 	</ul>
 
 <h3 id='basic-shape-serialization'>