[css-fonts-4] Specify the interaction between font variations and synthesis

Follow up to 00a28c7

Author: litherum

css-fonts-4/Overview.bs

@@ -1595,6 +1595,8 @@ at least as dark as its predecessor. Only values between 1 - 999 are valid, and
 
 	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.
 
+	User agents must not synthesize optical sizing when it is not performed by the font directly.
+
 <h3>Low-level font variation settings control: the 'font-variation-settings' property</h3>
 
 	<pre class="propdef">