[css-logical-1] Add some concepts for logical properties for other specs to use

Author: upsuper

css-logical-1/Overview.bs

@@ -289,6 +289,22 @@ Flow-Relative Box Model Properties</h2>
   depends on the element’s computed values of
   'writing-mode', 'direction', and 'text-orientation'.
 
+  Each set of flow-relative longhand properties and their parallel
+  physical properties form a <dfn export>logical property group</dfn>.
+
+  Each property in a <a>logical property group</a> has
+  a <dfn export>logical kind</dfn>. Flow-relative properties'
+  <a>logical kind</a> is "flow-relative", and physical properties'
+  <a>logical kind</a> is "physical".
+
+  Note: For example, 'block-size', 'inline-size', 'width', and 'height'
+  form a <a>logical property group</a>, while 'margin-block-start',
+  'margin-block-end', 'margin-inline-start', 'margin-inline-end',
+  'margin-top', 'margin-right', 'margin-bottom', and 'margin-left'
+  form another.
+
+  Note: Each property is in at most one <a>logical property group</a>.
+
   Note: Depending on the element’s own writing mode
   simplifies the cascading calculations
   and gives a straightforward model for authors to reason about.