[css-box-3] Define all the box keywords in the box spec. Relates to w3c/fxtf-drafts#66

Author: fantasai

css-box-3/Overview.bs

@@ -193,6 +193,58 @@ The CSS Box Model {#box-model}
 	is specified in [[css-break-3]]
 	and controlled by the 'box-decoration-break' property.
 
+Box-edge Keywords {#keywords}
+-----------------
+
+	The following CSS keywords are defined for use
+	in properties (such as 'transform-box' and 'background-clip')
+	that need to refer to various box edges:
+
+	<dl dfn-for="<box>,<shape-box>,<geometry-box>" dfn-type=value>
+		<dt><dfn>content-box</dfn>
+			<dd>
+				Refers to the [=content box=] or [=content edge=].
+				(In an SVG context, treated as ''<box>/fill-box''.)
+
+		<dt><dfn>padding-box</dfn>
+			<dd>
+				Refers to the [=padding box=] or [=padding edge=].
+				(In an SVG context, treated as ''<box>/fill-box''.)
+
+		<dt><dfn>border-box</dfn>
+			<dd>
+				Refers to the [=border box=] or [=border edge=].
+				(In an SVG context, treated as ''<box>/stroke-box''.)
+
+		<dt><dfn>margin-box</dfn>
+			<dd>
+				Refers to the [=margin box=] or [=margin edge=].
+				(In an SVG context, treated as ''<box>/stroke-box''.)
+
+		<dt><dfn>fill-box</dfn>
+			<dd>
+				Refers to the [=object bounding box=] or its edges.
+				(In a CSS box context, treated as ''<box>/content-box''.)
+
+		<dt><dfn>stroke-box</dfn>
+			<dd>
+				Refers to the [=stroke bounding box=] or its edges.
+				(In a CSS box context, treated as ''<box>/border-box''.)
+
+		<dt><dfn>view-box</dfn>
+			<dd>
+				Refers to the nearest [=SVG viewport=].
+				(In a CSS box context, treated as ''<box>/border-box''.)
+	</dl>
+
+	For convenience, the following value types are defined:
+	<pre>
+		<dfn><<visual-box>></dfn> = content-box | padding-box | border-box
+		<dfn><<layout-box>></dfn> = content-box | padding-box | border-box | margin-box
+		<dfn><<paint-box>></dfn> = content-box | padding-box | border-box | fill-box | stroke-box
+		<dfn><<coord-box>></dfn> = content-box | padding-box | border-box | fill-box | stroke-box | view-box
+	</pre>
+
 Margins {#margins}
 =======