[css-fonts-4] Define synthetic oblique in vertical text, fix #2869

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2031,6 +2031,25 @@ Controlling synthesized oblique: The 'font-synthesis-style' property</h4>
 	        Synthesis of oblique faces is not allowed
 	</dl>
 
+	<p id="vertical-oblique-synthesis">
+		In vertical text,
+		for positive oblique angles,
+		the glyph is skewed 
+		such that the line-over edge shifts towards the line-right side 
+		while the line-under edge shifts towards the line-left side.
+		For negative oblique angles,
+		the glyph is skewed 
+		such that the line-over edge shifts towards the line-left side 
+		while the line-under edge shifts towards the line-right side.
+		Skewing is about the center of the glyph.
+	</p>
+
+	<!-- TODO make a more visually pleasing figure -->
+	<figure>
+		<img alt="synthetic oblique in vertical text" src="images/italics-vertical.png" width="78" >
+		<figcaption>Synthetic oblique in vertical text, positive oblique angle</figcaption>
+	</figure>
+
 <h4 id="font-synthesis-small-caps">
 Controlling synthesized small caps: The 'font-synthesis-small-caps' property</h4>