[css-fonts-4] Added oblique-only as a value of font-synthesis-style w3c#9390

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2075,7 +2075,7 @@ Controlling synthesized oblique: The 'font-synthesis-style' property</h4>
 
 	<pre class="propdef">
 	Name: font-synthesis-style
-	Value: auto | none
+	Value: auto | none | oblique-only
 	Initial: auto
 	Applies to: all elements and text
 	Inherited: yes
@@ -2106,6 +2106,12 @@ Controlling synthesized oblique: The 'font-synthesis-style' property</h4>
 	    <dt><dfn>none</dfn>
 	    <dd>
 	        Synthesis of oblique faces is not allowed
+
+		<dt><dfn>oblique-only</dfn>
+			<dd>
+				Synthesis of oblique faces is allowed,
+				but they must not be used as fallback
+				if italic is specified
 	</dl>
 
 	<p id="vertical-oblique-synthesis">