[css-inline-3] sub/super font metrics are a MAY; define fallback ratios. #5225

Author: fantasai

css-inline-3/Overview.bs

@@ -851,12 +851,18 @@ Post-Alignment Shift: the 'baseline-shift' longhand</h4>
 		<dd>Raise (positive value) or lower (negative value) by the specified percentage of the 'line-height'.
 
 		<dt><dfn>sub</dfn>
-		<dd>Lower by the offset appropriate for subscripts of the parent’s box.
-		(The UA should use the parent’s font data to find this offset whenever possible.)
+		<dd>
+			Lower by the offset appropriate for subscripts of the parent’s box.
+			The UA may use the parent’s font metrics to find this offset;
+			otherwise it defaults to dropping by
+			one fifth of the parent’s used 'font-size'.
 
 		<dt><dfn>super</dfn>
-		<dd>Raise by the offset appropriate for superscripts of the parent’s box.
-		(The UA should use the parent’s font data to find this offset whenever possible.)
+		<dd>
+			Raise by the offset appropriate for superscripts of the parent’s box.
+			The UA may use the parent’s font metrics to find this offset;
+			otherwise it defaults to raising by
+			one third of the parent’s used 'font-size'.
 
 		<dt><dfn>top</dfn>
 		<dd>