[css-fonts-4] Add note suggesting heuristics. w3c#4108

Author: tabatkins

css-fonts-4/Overview.bs

@@ -2888,6 +2888,25 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 			but ''optional'' ensures they can actually <em>use</em> the site,
 			rather than quitting and going elsewhere because the site takes too long to load.
 		</div>
+
+		<div class=note>
+			To increase the chance that an ''optional'' font will be available
+			in time to use for rendering some text,
+			it is recommended that the user agent employ heuristics
+            to make it faster to access
+            or to judge when it would be worthwhile to delay rendering some text,
+			such as checking if the font is preloaded in HTML,
+			or moving the file from slower to faster in-memory caches
+			as soon as it is seen in a stylesheet
+			(before it's known whether it will be used on the page).
+
+			These heuristics cannot be relied upon by authors, however;
+			it must be understood that the ''optional'' value
+			can result in the font never being used.
+			If a somewhat higher assurance of the font being used is needed,
+			authors should consider using the ''fallback'' value.
+		</div>
+
 </dl>