[css-display] Add a section defining element/box/fragment.

Author: tabatkins

css-display/Overview.bs

@@ -12,8 +12,7 @@ Previous Version: http://www.w3.org/TR/2014/WD-css-display-3-20140220/
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
 Editor: fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
 Abstract: This module describes how the CSS formatting box tree is generated from the document element tree and defines the 'display' and 'box-suppress' properties that control it.
-Ignored Terms: display-model, display-role, table row group box, ruby container
-Link Defaults: css-lists-3 (property) counter-increment
+Ignored Terms: display-model, display-role, table row group box
 </pre>
 
 <h2 id="intro">
@@ -506,8 +505,64 @@ Run-In Layout</h2>
 
 	Note: This run-in model is slightly different from the one proposed in earlier revisions of [[!CSS21]].
 
+<h2 id='element-box-fragment'>
+Appendix A: Elements, Boxes, and Fragments</h2>
+
+	CSS operates on several different types of objects,
+	depending on the stage of processing and what is being done:
+	the <a>element</a>, the <a>box</a>, and the <a>fragment</a>.
+
+	Note: Many of the CSS specs were written before this terminology was ironed out,
+	or refer to things incorrectly,
+	so view older specs with caution when they're using these terms.
+	It should be possible to infer from context which term they really mean.
+	Please report errors in specs when you find them,
+	so they can be corrected.
+
+	An <dfn>element</dfn> is an object in the DOM tree returned by the host element.
+	The purpose of an <a>element</a> is to receive styling,
+	so the multiple declarations from the document's style sheets
+	are converted into values for each property on each element.
+	(The DOM tree also contains text,
+	which is identical to <a>elements</a>
+	except that it can't be styled directly,
+	only through inheritance.)
+
+	<a>Elements</a> generate <a>boxes</a>.
+	Typically, an <a>element</a> generates a single <a>box</a>.
+	Some properties (such as ''display:none'') cause an <a>element</a> and/or its descendants to not generate any <a>boxes</a> at all.
+	Some properties (such as ''columns: 2'') cause an <a>element</a> to generate multiple <a>boxes</a>s.
+	Some combinations of properties (such as an element with ''display:table-cell'' and a parent element with anything but ''display:table-row'')
+	trigger the creation of <a>anonymous boxes</a> to keep the <a>box tree</a> reasonable.
+
+	A <dfn>box</dfn> is an object generated from an <a>element</a> according to the 'display' property
+	and other layout-affecting properties,
+	organized into a <dfn>box tree</dfn>.
+	The purpose of a <a>box</a> is to organize the document into a structure that is suitable for layout.
+	<a>Boxes</a> have the same styles as their generating <a>element</a>, unless otherwise indicated.
+	They're referred by their type, typically determined by the 'display' value--
+	a <a>box</a> generated by an element with ''display: block'' is called a “block box” or just a “block”.
+
+	An <dfn>anonymous box</dfn> is generated in certain circumstances
+	where a type of <a>box</a> (such as a table cell box)
+	requires a particular type of parent box (such as a table row box)
+	but the document structure is not written to produce that--
+	<a>anonymous boxes</a> are then generated between the parent and child to provide the required structures.
+	Anonymous boxes are styled as if they inherited from their nearest non-anonymous parent <a>box</a>,
+	unless otherwise specified.
+
+	The process of layout generates <a>fragments</a> from <a>boxes</a>.
+
+	A <dfn>fragment</dfn> is an object representing the result of layout,
+	organized into a <dfn>fragment tree</dfn>.
+	<a>Fragments</a> have a size and position,
+	and the same styles as their generating <a>boxes</a>
+	unless otherwise specified.
+
+
+
 <h2 id='glossary'>
-Glossary</h2>
+Appendix B: Glossary</h2>
 
 	The following terms are defined here for convenience: