[css-fonts-4] [varfont] Define font-optical-sizing's relationship with zooming

See w3c#538

Author: Litherum

css-fonts-4/Overview.bs

@@ -1143,8 +1143,6 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 
 <h2 id="font-variation-props">Font Variation Properties</h2>
 
-	Issue: For all the various kinds of zooms, describe whether font-optical-sizing is re-applied (layout-time) or not (paint scaling).
-
 	<pre class="propdef">
 	Name: font-optical-sizing
 	Value: auto | none
@@ -1166,6 +1164,10 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 		<dd>The user agent must not modify the shape of glyphs for optical size.</dd>
 	</dl>
 
+	Pixel density as well as visual size of the text may influence the variation value chosen for 'font-optical-sizing'. When either pixel density or visual size of the text changes in response to a user operation or style change, user agents may choose a new value for this variation value only if the change is layout-causing. User agents are free to determine which changes are layout-causing.
+
+	Note: Some user operations, such as pinch-zoom, may be considered not-layout-causing if they do not cause text to reflow. However, other user operations, such as increasing text size for accessiblity purposes, may be considered layout-causing because they cause text to reflow. Similarly, the 'transform' property may be considered not-layout-causing because transforms generally do not cause text to reflow. Each user-agent is free to decide whether or not each operation is layout-changing or not.
+
 	<div class="figure"><img alt="text with and without 'font-optical-size'" src="fontopticalsize.png" /><p class="caption">Text with and without the use of 'font-optical-size'</p></div>
 
 <h3>Low-level font variation settings control: the 'font-variation-settings' property</h3>