[css-fonts-4] [varfont] Add a section describing ambiguity of font selection

Author: Litherum

css-fonts-4/Overview.bs

@@ -894,6 +894,11 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 		</li>
 
 		</ol>
+		Note that more than one font may be remaining in the matching set after performing
+		the above steps. If so, the user agent must choose a single font from
+		the matching set and continue these steps with it. The choice of which
+		font to choose may differ between multiple user agents and multiple operating
+		system platforms; however, it must not differ between two elements in the same document.
 		</li>
 
 		<li><p>If the matched face is defined via <code>@font-face</code>