[css-anchor-position] Added the standard Value Definitions section, so it could hold the note about the flat tree. w3c#10325

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -104,6 +104,21 @@ This can be done with the following CSS:
 	</pre>
 </div>
 
+<h3 id="values">
+Value Definitions</h3>
+
+This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
+using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
+Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
+Combination with other CSS modules may expand the definitions of these value types.
+
+In addition to the property-specific values listed in their definitions,
+all properties defined in this specification
+also accept the <a>CSS-wide keywords</a> as their property value.
+For readability they have not been repeated explicitly.
+
+Like most operations in CSS besides selector matching, features in this specification operate over the flattened element tree.
+
 Determining the Anchor {#determining}
 ======================