[css-backrounds-3] Explicitly mention continuity as rationale for radius adjustment formula. w3c#1900.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2018,7 +2018,8 @@ the inner curve will not be a full quarter ellipse.</p>
 
 <p>The margin edge, being outside the border edge, calculates its radius
 by <em>adding</em> the corresponding margin thickness to each border radius.
-However, in order to create a sharper corner when the border radius is small,
+However, in order to create a sharper corner when the border radius is small
+(and thus ensure continuity between round and sharp corners),
 when the <i>border radius</i> is less than the margin,
 the margin is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
 where <var>r</var> is the ratio of the border radius to the margin,
@@ -2932,7 +2933,8 @@ Shadow Shape, Spread, and Knockout</h4>
   <p>To preserve the box's shape when spread is applied,
   the corner radii of the shadow are also increased (decreased, for inner shadows)
   from the border-box radii by adding the <i>spread distance</i> (and flooring at zero).
-  However, in order to create a sharper corner when the border radius is small,
+  However, in order to create a sharper corner when the border radius is small
+  (and thus ensure continuity between round and sharp corners),
   when the <i>border radius</i> is less than the <i>spread distance</i>,
   the <i>spread distance</i> is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
   where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,