[css-values-4] Improve highest-level description for viewport units.

Author: fantasai

css-values-4/Overview.bs

@@ -1743,7 +1743,10 @@ Viewport-percentage Lengths: the ''*vw'', ''*vh'', ''*vi'', ''*vb'', ''*vmin'',
 	as an implementation reference for now.
 
 	The <dfn export>viewport-percentage lengths</dfn> are relative to the size of the
-	<a href="https://www.w3.org/TR/CSS21/visudet.html#containing-block-details">initial containing block</a>.
+	<a href="https://www.w3.org/TR/CSS21/visudet.html#containing-block-details">initial containing block</a>--
+	which is itself based on the size of either
+	the viewport (for [=continuous media=])
+	or the [=page area=] (for [=paged media=]).
 	When the height or width of the initial containing block is changed,
 	they are scaled accordingly.
 
@@ -1859,11 +1862,6 @@ The Large, Small, and Dynamic Viewport Sizes</h5>
 	(and therefore have no effect on any of the [=viewport-percentage lengths=];
 	typically on-screen keyboards will fit into this category.
 
-	For [=paged media=], the exact definition of the viewport-percentage lengths
-	is deferred to [[!CSS3PAGE]].
-	However, in the typical case of all pages being the same size,
-	they are sized relative to the [=initial containing block=].
-
 	In all cases, scrollbars are assumed not to exist.
 	<span class='note'>Note however that the [=initial containing block=]'s size
 	<em>is</em> affected by the presence of scrollbars on the viewport.</span>