[css-display] Specify how styles reflect back from boxes to their generating elements. Fixes #1643.

Author: tabatkins

css-display-3/Overview.bs

@@ -15,7 +15,7 @@ Previous Version: https://www.w3.org/TR/2014/WD-css-display-3-20140911/
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Abstract: This module describes how the CSS formatting box tree is generated from the document element tree and defines the 'display' property that controls it.
-Ignored Terms: display-model, display-role, table row group box, absolutely positioned, table formatting context
+Ignored Terms: display-model, display-role, table row group box, absolutely positioned, table formatting context, table grid box
 At Risk: Application of ''::first-letter'' in the presence of run-ins
 At Risk: ''display: run-in''
 At Risk: All multi-keyword values of 'display'
@@ -70,10 +70,26 @@ Introduction</h2>
 	And some values
 	(such as ''display/none'' or ''display/contents'')
 	cause the [=CSS/element=] and/or its descendants to not generate any <a>boxes</a> at all.
-	<a>Boxes</a> are assigned the same styles as their generating [=CSS/element=], unless otherwise indicated.
-	They're often referred to by their 'display' type--
+	<a>Boxes</a> are often referred to by their 'display' type--
 	e.g. a <a>box</a> generated by an element with ''display: block'' is called a “block box” or just a “block”.
 
+	A <a>box</a> is assigned the same styles as its generating [=CSS/element=], unless otherwise indicated.
+	In general, <a>inherited properties</a> are assigned to the <a>principal box</a>,
+	and then inherit through the box tree
+	to any other boxes generated by the same element.
+	Non-inherited properties default to applying to the <a>principal box</a>,
+	but when the element generates multiple boxes,
+	are sometimes defined to apply to a different box:
+	for example, the 'border' properties applied to a table element
+	are applied to its <a>table grid box</a>,
+	not to its <a lt="principal box">principal</a> <a>table wrapper box</a>.
+	If the value computation process alters the styles of those boxes,
+	and the element's style is requested
+	(such as through {{getComputedStyle()}}),
+	the element reflects,
+	for each property,
+	the value from the box to which that property was applied.
+
 	Similarly, each contiguous sequence of sibling <a>text nodes</a> generates a <a>text run</a>,
 	which is assigned the same styles as the generating <a>text nodes</a>.