[css-writing-modes] Clarified baseline of upright glyphs

Author: kojiishi

css-writing-modes/Overview.bs

@@ -1641,6 +1641,23 @@ Line-relative Directions</h3>
     </div>
   </div>
 
+  <div class="note">
+    <div class="sidefigure">
+      <p><img src="vertical-upright-baseline.svg"
+        alt="Baseline of an upright glyph is drawn vertically from the top center"
+        width=80>
+      <p class="caption">Vertical baseline of an upright glyph
+    </div>
+    When ''text-orientation: upright'',
+    the baseline is still vertical,
+    and the vertical baseline in the font is used,
+    or vertical baseline is synthesized if the font does not provide.
+  <p>
+    Since the baseline is vertical,
+    the definitions for ''sideways-right'' still applies; i.e.,
+    <a>line-over</a> is on right, and <a>line-under</a> is on left.
+  </div>
+
 <h3 id="logical-to-physical">
 Abstract-to-Physical Mappings</h3>