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

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2018,7 +2018,8 @@ the inner curve will not be a full quarter ellipse.
 
 The margin edge, being outside the border edge, calculates its radius
 by adding 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 border radius is less than the margin,
 the margin is multiplied by the proportion 1 + (r-1)3,
 where r is the ratio of the border radius to the margin,
@@ -2932,7 +2933,8 @@ Shadow Shape, Spread, and Knockout
   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 spread distance (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 border radius is less than the spread distance,
   the spread distance is multiplied by the proportion 1 + (r-1)3,
   where r is the ratio of the border radius to the spread distance,