[css-backgrounds] Add note about 3D border color calculations as requested in https://lists.w3.org/Archives/Public/www-style/2015Sep/0252.html

Author: fantasai

css-backgrounds/Overview.src.html

@@ -1677,6 +1677,10 @@ <h3 id="the-border-style">Line Patterns: the 'border-style' properties</h3>
   <p class=caption>Example renderings of the predefined border styles.
 </div>
 
+<p class="note">Note: Border colors close to black or white
+may need different color calculations than colors in between
+in order to create the required “3D” effect.
+
 <p class="note">Note: There is no control over the spacing of the dots
 and dashes, nor over the length of the dashes. Implementations are
 encouraged to choose a spacing that makes the corners symmetrical.
@@ -1686,8 +1690,6 @@ <h3 id="the-border-style">Line Patterns: the 'border-style' properties</h3>
 rounded corners may cause the corners and the contents to overlap, if
 the padding is less than the radius of the corner.
 
-
-
 <h3 id="the-border-width">Line Thickness: the 'border-width' properties</h3>
 
 <table class="propdef">