[css-display] Add spec-author guidance for box construction, per #1643.

css-display-3/Overview.bs

@@ -1169,6 +1169,27 @@ SVG Elements {#unbox-svg}
 	where their contents are handled in a totally separate context from their parent,
 	just removes itself entirely.
 
+<h2 class=no-num id="box-guidelines">
+Appendix C: Box Construction Guidelines for Spec Authors</h2>
+
+	<em>This section is non-normative guidance for specification authors.</em>
+
+	* A box cannot be <a>blockified</a> and <a>inlinified</a> at the same time;
+		if such a thing would occur,
+		define which wins over the other.
+	* Non-principal non-anonymous boxes can't be <a>blockified</a>:
+		blockification affects the element’s computed values
+		and thus determines the type of its <a>principal box</a>.
+	* Boxes which <a>blockify</a> their contents
+		can't directly contain <a>inline-level</a> content;
+		any boxes or text runs generated within such an element
+		must be <a>blockified</a>
+		or wrapped in an <a>anonymous</a> <a>block container</a>.
+	* Boxes which <a>inlinify</a> their contents
+		can't directly contain <a>block-level</a> boxes;
+		any boxes generated within such an element
+		must be <a>inline-level</a>.
+
 <h2 class=no-num id="acknowledgments">
 Acknowledgments</h2>