Better wording for transition region image

Author: fantasai

css3-background/Overview.html

@@ -2414,16 +2414,18 @@ <h3 id=the-border-radius><span class=secno>4.4 </span>The &lsquo;<code
    borders.
 
   <div class=figure>
-   <p><img alt="The transition region on curved corners"
+   <p><img alt="Illustration of the transition region on curved corners"
     src=transition-region.png>
 
    <p class=caption>Given these corner shapes, color and style transitions
     must be contained within the green region. In case D the rectangle
     defined by the border radii does not include the center of the inner
     curve (which is a sharp corner), so the transition region is expanded to
-    include that corner. In cases like this, color transitions look best when
-    restricted to the dark green region (defined by the tips of the border
-    radii and the center of the inner curve).</p>
+    include that corner. Transitions may take up the entire transition
+    region, but are not required to: For example, a gradient color transition
+    between two solid border styles might take up only the region bounded by
+    the tips of the outer radii and the center of the inner curve
+    (represented in case D by the dark green region).
   </div>
 
   <p>Corner curves must not overlap: When the sum of two adjacent corner

css3-background/Overview.src.html

@@ -1752,16 +1752,18 @@ <h3 id="the-border-radius">The 'border-radius' properties</h3>
 borders.
 
 <div class="figure">
-  <p><img alt="The transition region on curved corners"
+  <p><img alt="Illustration of the transition region on curved corners"
   src="transition-region.png">
 
   <p class=caption>Given these corner shapes, color and style transitions
     must be contained within the green region. In case D the rectangle
     defined by the border radii does not include the center of the inner
     curve (which is a sharp corner), so the transition region is expanded
-    to include that corner. In cases like this, color transitions look
-    best when restricted to the dark green region (defined by the tips of
-    the border radii and the center of the inner curve).</p>
+    to include that corner. Transitions may take up the entire transition
+    region, but are not required to: For example, a gradient color transition
+    between two solid border styles might take up only the region bounded
+    by the tips of the outer radii and the center of the inner curve
+    (represented in case D by the dark green region).
 </div>
 
 <p>Corner curves must not overlap: When the sum of two adjacent corner radii