[css-fonts-3] Help with #1959, still needs WPT

Author: svgeesus

css-fonts-3/Fonts.src.html

@@ -4168,6 +4168,9 @@ <h3 id="font-feature-settings-prop">Low-level font feature settings control: the
 synonymous with 0. If the value is omitted, a value of 1 is assumed.
 </p>
 
+<p id="ffs-map">The computed value of font-feature-settings is a map,
+  so any duplicates in the specified value must not be preserved. </p>
+
 <pre>
 font-feature-settings: "dlig" 1;       /* dlig=1 enable discretionary ligatures */
 font-feature-settings: "smcp" on;      /* smcp=1 enable small caps */