[css-inline-3] Define central baseline in reference to already-defined text-under/text-over baselines.

Author: fantasai

css-inline-3/Overview.bs

@@ -228,7 +228,7 @@ Baseline Metrics</h3>
 		<dt><dfn lt="x-middle baseline" local-lt="x-middle">x-middle</dfn>
 		<dd>
 			Corresponds to halfway between
-			the [=alphabetic=] and [=x-height=] metrics.
+			the [=alphabetic=] and [=x-height=] baselines.
 
 		<dt><dfn lt="ideographic-over baseline" local-lt="ideographic-over">ideographic-over</dfn>
 		<dd>
@@ -242,13 +242,6 @@ Baseline Metrics</h3>
 			the [=line-under=] design edge of CJK (Han/Hangul/Kana) text.
 			Assigned <code>ideo</code> in OpenType.
 
-		<dt><dfn lt="central baseline" local-lt="central">central</dfn>
-		<dd>
-			Corresponds to halfway between
-			...
-
-			ISSUE: [ the [=ideographic-over=] and [=ideographic-under=] baselines | the ascender and descender edges of the em box ] ?
-
 		<dt><dfn lt="ideographic-ink-over baseline" local-lt="ideographic-ink-over">ideographic-ink-over</dfn>
 		<dd>
 			Corresponds to the [=line-over=] ink edge of CJK (Han/Hangul/Kana) text.
@@ -281,6 +274,15 @@ Baseline Metrics</h3>
 		<dd>
 			Corresponds to the metric used as the [=line-under=] edge
 			of an [=inline box|inline=]’s [=content box=] per [[CSS2]].
+
+
+		<dt><dfn lt="central baseline" local-lt="central">central</dfn>
+		<dd>
+			Corresponds to halfway between
+			the [=text-under=] and [=text-over=] baselines.
+
+			ISSUE(5177): This is the default baseline in [=vertical writing modes=].
+			Can we make it interoperable? Or do we need another value that is?
 	</dl>
 
 	Note: These metrics are optical design metrics,