[css-cascade] Add note about how 'computed value' definitions are *not* serialization definitions. Fixes w3c#4060.

Author: tabatkins

css-cascade-3/Overview.bs

@@ -376,7 +376,13 @@ Computed Values</h3>
 
 	Note: The <a>computed value</a> is the value that is transferred from parent to child during <a>inheritance</a>.
 	For historical reasons,
-	it is not necessarily the value returned by the <code>getComputedStyle()</code> function.
+	it is not necessarily the value returned by the {{getComputedStyle()}} function,
+	which sometimes returns <a>used values</a>. [[CSSOM]]
+	Furthermore, the <a>computed value</a> is an abstract data representation:
+	their definitions reflect that data representation,
+	not how that data is serialized.
+	For example, serialization rules often allow omitting certain values which are implied during parsing;
+	but those values are nonetheless part of the <a>computed value</a>.
 
 	<div class="example">
 		A <a>specified value</a> can be either absolute (i.e., not relative to another value, as in ''red'' or ''2mm'')

css-cascade-4/Overview.bs

@@ -462,9 +462,17 @@ Computed Values</h3>
 	as defined in the “Computed Value” line of the property definition table,
 	generally absolutizing it in preparation for <a>inheritance</a>.
 
-	Note: The <a>computed value</a> is the value that is transferred from parent to child during <a>inheritance</a>.
-	For historical reasons,
-	it is not necessarily the value returned by the <code>getComputedStyle()</code> function.
+	<div class=note>
+		The <a>computed value</a> is the value that is transferred from parent to child during <a>inheritance</a>.
+		For historical reasons,
+		it is not necessarily the value returned by the {{getComputedStyle()}} function,
+		which sometimes returns <a>used values</a>. [[CSSOM]]
+		Furthermore, the <a>computed value</a> is an abstract data representation:
+		their definitions reflect that data representation,
+		not how that data is serialized.
+		For example, serialization rules often allow omitting certain values which are implied during parsing;
+		but those values are nonetheless part of the <a>computed value</a>.
+	</div>
 
 	<div class="example">
 		A <a>specified value</a> can be either absolute (i.e., not relative to another value, as in ''red'' or ''2mm'')