[css-box-3] Add cross-reference section on borders, for parallelism. w3c#3007

Author: fantasai

css-box-3/Overview.bs

@@ -201,7 +201,8 @@ Margins {#margins}
 	sets the margin for all four sides
 	while the margin <a>longhand properties</a> only set their respective side.
 	This specification defines the <a>physical</a> 'margin' <a>longhands</a>;
-	[[css-logical-1]] additionally defines <a>flow-relative</a> 'margin' <a>longhands</a>.
+	[[css-logical-1#margin-properties]] additionally
+	defines <a>flow-relative</a> 'margin' <a>longhands</a>.
 	Both sets of properties control the same set of margins:
 	they are just different ways of indexing each side.
 
@@ -383,7 +384,8 @@ Padding {#paddings}
 	sets the padding for all four sides
 	while the padding <a>longhand properties</a> only set their respective side.
 	This specification defines the <a>physical</a> 'padding' <a>longhands</a>;
-	[[css-logical-1]] additionally defines <a>flow-relative</a> 'padding' <a>longhands</a>.
+	[[css-logical-1#padding-properties]] additionally
+	defines <a>flow-relative</a> 'padding' <a>longhands</a>.
 	Both sets of properties control the same set of padding:
 	they are just different ways of indexing each side.
 
@@ -461,6 +463,21 @@ Padding Shorthand: the 'padding' property {#padding-shorthand}
 		</pre>
 	</div>
 
+Borders {#borders}
+=======
+
+	<dfn export lt="border">Borders</dfn> fill the <a>border area</a>,
+	to visually delineate the edges of the box,
+	The <dfn export>border properties</dfn> specify the thickness
+	of the <a>border area</a> of a box,
+	as well as its drawing style and color.
+	See [[css-backgrounds-3#borders]] for the definition
+	of the physical variants of these properties;
+	[[css-logical-1#border-properties]] additionally
+	defines <a>flow-relative</a> border <a>longhands</a>.
+	Both sets of properties control the same set of borders:
+	they are just different ways of indexing each side.
+
 Changes Since CSS Level 2 {#changes}
 =========================