[css-fonts-5] vertical metrics overrides

Fixes #6152.

Author: litherum

css-fonts-5/Overview.bs

@@ -149,21 +149,21 @@ the 'ascent-override', 'descent-override', 'line-gap-override', and 'size-adjust
 
 <pre class='descdef'>
 Name: ascent-override
-Value: normal | <<percentage>>
+Value: normal | <<percentage>>{1,2}
 For: @font-face
 Initial: normal
 </pre>
 
 <pre class='descdef'>
 Name: descent-override
-Value: normal | <<percentage>>
+Value: normal | <<percentage>>{1,2}
 For: @font-face
 Initial: normal
 </pre>
 
 <pre class='descdef'>
 Name: line-gap-override
-Value: normal | <<percentage>>
+Value: normal | <<percentage>>{1,2}
 For: @font-face
 Initial: normal
 </pre>
@@ -180,6 +180,15 @@ The 'ascent-override', 'descent-override', and 'line-gap-override' descriptors d
 <a spec="CSS-INLINE-3">line gap metric</a> of the font, respectively. The `size-adjust`
 descriptor defines a scalar by which glyph outlines and metrics from this font are multiplied.
 
+The first value to 'ascent-override', 'descent-override', or 'line-gap-override' is used for
+vertical distance with upright glyph orientation (e.g. <a>horizontal writing modes</a> or
+<a>vertical writing modes</a> with <code>text-orientation: mixed</code>) and horizontal
+distance with sideways glyph orientation (e.g. <a>vertical writing modes</a>
+with <code>text-orientation: sideways</code>. The second value to these descriptors, if it
+is present, is used for horizontal distance with upright glyph orientation in <a>vertical writing
+modes</a>. If only one value is present, the descriptor behaves as if that one value was repeated
+twice.
+
 When the descriptor value is 'normal', the corresponding metric value is obtained as-if the
 descriptor was absent from the <code>@font-face</code> block.
 
@@ -274,14 +283,14 @@ Issue(5518):
 
     <pre class='descdef'>
         Name: superscript-position-override
-        Value: normal | <<percentage>>
+        Value: normal | <<percentage>>{1,2}
         For: @font-face
         Initial: normal
     </pre>
 
     <pre class='descdef'>
         Name: subscript-position-override
-        Value: normal | <<percentage>>
+        Value: normal | <<percentage>>{1,2}
         For: @font-face
         Initial: normal
     </pre>
@@ -300,7 +309,14 @@ Issue(5518):
         Initial: normal
     </pre>
 
-
+The first value to 'superscript-position-override' and 'subscript-position-override' is used for
+vertical distance with upright glyph orientation (e.g. <a>horizontal writing modes</a> or
+<a>vertical writing modes</a> with <code>text-orientation: mixed</code>) and horizontal
+distance with sideways glyph orientation (e.g. <a>vertical writing modes</a>
+with <code>text-orientation: sideways</code>. The second value to these descriptors, if it
+is present, is used for horizontal distance with upright glyph orientation in <a>vertical writing
+modes</a>. If only one value is present, the descriptor behaves as if that one value was repeated
+twice.
 
 <h2 id="font-rend-props">
 Font Feature Properties</h2>