[css-values] Resolve issue about using vi/vb in MQs by using the init…

…ial value of WM, as discussed in #113. Fixes #113.

css-values/Overview.bs

@@ -1041,10 +1041,11 @@ Viewport-percentage lengths: the ''vw'', ''vh'', ''vmin'', ''vmax'' units</h4>
 			Equal to the larger of ''vw'' or ''vh''.
 	</dl>
 
-	Issue: All units need to be possible to evaluate in a context without any elements or styling;
-	for example, ''rem'' in a media query is evaluated
-	relative to the document's initial font size.
-	How do we get an inline/block direction from the document?
+	In situations where there is no root element
+	or it hasn't yet been styled
+	(such as when evaluating <a>media queries</a>),
+	the ''vi'' and ''vb'' units use the initial value of the 'writing-mode' property
+	to determine which axis they correspond to.
 
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