[css-fonts-4] Further clarify the behavior of 'optional' to stress that the important thing is to avoid layout jank at all costs. Fixes w3c#4108.

Author: tabatkins

css-fonts-4/Overview.bs

@@ -2792,22 +2792,25 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 
 	<dt><dfn>optional</dfn>
 	<dd>
-		Gives the font face an extremely small <a>block period</a>
-		(''100ms'' or less is recommended in most cases)
-		and a ''0s'' <a>swap period</a>.
+		If the font can be loaded "immediately"
+		(such that it's available to be used for the "first paint" of the text),
+		the font is used.
 
-		If the font is not retrieved before the two durations expire,
+		Otherwise, the font is treated
+		as if its [=block period=] and [=swap period=] both expired before it finished loading.
+		If the font is not used due to this,
 		the user agent may choose to abort the font download,
 		or download it with a very low priority.
 		If the user agent believes it would be useful for the user,
 		it may avoid even starting the font download,
 		and proceed immediately to using a fallback font.
 
-		Note: In other words, the font is used if it's already downloaded and available,
-		but otherwise a fallback is used for the rest of the page's lifetime instead.
-		The font might download in the background and be available to future page loads,
-		but if the user-agent detects that the user has very limited bandwidth,
-		it might choose to simply never download and use the font.
+		An ''optional'' font <strong>must never</strong> 
+		cause the layout of the page to "jump" as it loads in.
+		A user agent <em>may</em> choose to slightly delay rendering an element using an optional font
+		to give it time to load from a possibly-slow local cache,
+		but once the text has been painted to the screen with a fallback font instead,
+		it must not be rendered with the ''optional'' font for the rest of the page's lifetime.
 
 		This value should be used for body text,
 		or any other text where the chosen font is purely a decorative "nice-to-have".