[css-box-3] Fold in SVG weirdness. w3c#4662

Author: fantasai

css-box-3/Overview.bs

@@ -233,8 +233,20 @@ Box-edge Keywords {#keywords}
 
 		<dt><dfn>view-box</dfn>
 			<dd>
-				Refers to the nearest [=SVG viewport=].
+				Refers to the nearest [=SVG viewport=]’s
+				<dfn dfn lt="SVG viewport origin box">origin box</dfn>,
+				which is a rectangle with the width and height
+				of the [=SVG viewport=],
+				positioned such that its top left corner is anchored
+				at the coordinate system origin.
 				(In a CSS box context, treated as ''<box>/border-box''.)
+
+				Note: When the [=SVG viewport=]
+				is not itself anchored at the origin,
+				this [=origin box=] does not actually correspond
+				to the [=SVG viewport=]!
+				<!-- This is a legacy compat requirement.
+				     See https://github.com/w3c/csswg-drafts/issues/4662 -->
 	</dl>
 
 	For convenience, the following value types are defined: