Add the glossary section

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -970,6 +970,37 @@ to fully define the behavior.
 Issue(w3c/csswg-drafts#3376): The following currently does not account
 for <a href="https://github.com/WICG/overscroll-behavior">the proposed overscroll-behavior specification</a>.
 
+<h3 id=grouping>
+Glossary</h3>
+
+The following term definitions have been specified to explain the processing model for spatial navigation.
+See the links within the definitions for more information.
+
+The <dfn>boundary box</dfn> of an object is defined as follows:
+* if the object is a point, the boundary box is that point
+* if the object is a [=box=] or [=box fragment=], the boundary box is the <a>border box</a> of that box or fragment
+* if the object is a <a>focusable area</a> which is not an element, the boundary box is the axis-aligned the bounding box of that <a>focusable area</a>
+
+The <dfn>inside area</dfn> of an object is defined as follows:
+* if the object is a <a>scroll container</a>, its <a>optimal viewing region</a>
+* if the object is a <a>document</a>, the viewport of its <a>browsing context</a>
+* if the object is a [=box=] or [=box fragment=], its <a>boundary box</a>
+* otherwise, the <a>optimal viewing region</a> of its nearest ancestor <a>scroll container</a>
+
+NOTE: If an object is offscreen, the <a>inside area</a> should be the nearest visible ancestor container.
+
+Issue(w3c/csswg-drafts#2324): CSS should have a term for “border box taking into account corner shaping properties like border-radius”.
+
+The <dfn>search origin</dfn> is the origin for searching next target.
+
+The <dfn>spatial navigation starting point</dfn> is the origin for searching next target which is set by the User Agent. It is initially unset and it can be element or point.
+
+Note: For example, the User Agent could set it to the position of the user's click if the user clicks on the document contents,
+and unset when the focus is moved (by spatial navigation or any other means).
+
+If the User Agent sets both a <a>spatial navigation starting point</a> and a <a>sequential focus navigation starting point</a>,
+they must not be set differently.
+
 <h3 id=grouping>
 Groupings of elements</h3>
 
@@ -1099,31 +1130,6 @@ which would be bad for users.
 All geometrical operations in this section are defined to work on the result of CSS layout,
 including all graphical transformations, such as <a>relative positioning</a> or [[CSS-TRANSFORMS-1]].
 
-The <dfn>boundary box</dfn> of an object is defined as follows:
-* if the object is a point, the boundary box is that point
-* if the object is a [=box=] or [=box fragment=], the boundary box is the <a>border box</a> of that box or fragment
-* if the object is a <a>focusable area</a> which is not an element, the boundary box is the axis-aligned the bounding box of that <a>focusable area</a>
-
-The <dfn>inside area</dfn> of an object is defined as follows:
-* if the object is a <a>scroll container</a>, its <a>optimal viewing region</a>
-* if the object is a <a>document</a>, the viewport of its <a>browsing context</a>
-* if the object is a [=box=] or [=box fragment=], its <a>boundary box</a>
-* otherwise, the <a>optimal viewing region</a> of its nearest ancestor <a>scroll container</a>
-
-NOTE: If an object is offscreen, the <a>inside area</a> should be the nearest visible ancestor container.
-
-Issue(w3c/csswg-drafts#2324): CSS should have a term for “border box taking into account corner shaping properties like border-radius”.
-
-The <dfn>search origin</dfn> is the origin for searching next target.
-
-The <dfn>spatial navigation starting point</dfn> is the origin for searching next target which is set by the User Agent. It is initially unset and it can be element or point.
-
-Note: For example, the User Agent could set it to the position of the user's click if the user clicks on the document contents,
-and unset when the focus is moved (by spatial navigation or any other means).
-
-If the User Agent sets both a <a>spatial navigation starting point</a> and a <a>sequential focus navigation starting point</a>,
-they must not be set differently.
-
 <div algorithm="to set the search origin">
 
 To <dfn lt="set the search origin | setting the search origin">set the <a>search origin</a></dfn>,