[css-box-3] Update view-box definition with Amelia's wording in #4662 (comment)

Author: fantasai

css-box-3/Overview.bs

@@ -235,18 +235,19 @@ Box-edge Keywords {#keywords}
 
 		<dt><dfn>view-box</dfn>
 			<dd>
-				Refers to the nearest [=SVG viewport=]’s
+				Refers to the nearest [=SVG viewport=] element’s
 				<dfn dfn lt="SVG viewport origin box" local-lt="origin box">origin box</dfn>,
 				which is a rectangle with the width and height
-				of the [=SVG viewport=],
+				of the initial SVG <a spec=svg2>user coordinate system</a>
+				established by the <{svg/viewBox}> attribute for that element,
 				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,
+				Note: When the <{svg/viewBox}>
+				includes non-zero <var ignore>min-x</var> or <var ignore>min-y</var> offsets,
 				this [=origin box=] does not actually correspond
-				to the [=SVG viewport=]!
+				to the visible region rectangle defined by the <{svg/viewBox}>!
 				<!-- This is a legacy compat requirement.
 				     See https://github.com/w3c/csswg-drafts/issues/4662 -->
 	</dl>