[css-fonts-5] Define superscript/subscript override descriptors. w3c#5518

Author: fantasai

css-fonts-5/Overview.bs

@@ -278,37 +278,56 @@ the 'ascent-override', 'descent-override', and 'line-gap-override' descriptors</
 Superscript and subscript metrics overrides:
 the 'superscript-position-override', 'subscript-position-override','superscript-size-override' and 'subscript-size-override' descriptors</h3>
 
-Issue(5518):
-
-    <pre class='descdef'>
-        Name: superscript-position-override
-        Value: normal | <<percentage>>
-        For: @font-face
-        Initial: normal
-    </pre>
-
-    <pre class='descdef'>
-        Name: subscript-position-override
-        Value: normal | <<percentage>>
-        For: @font-face
-        Initial: normal
-    </pre>
-
-    <pre class='descdef'>
-        Name: superscript-size-override
-        Value: normal | <<percentage [0,Infinity]>>
-        For: @font-face
-        Initial: normal
-    </pre>
-
-    <pre class='descdef'>
-        Name: subscript-size-override
-        Value: normal | <<percentage [0,Infinity]>>
-        For: @font-face
-        Initial: normal
-    </pre>
+	<pre class='descdef'>
+	Name: superscript-position-override
+	Value: auto | from-font | <<percentage>>
+	For: @font-face
+	Initial: normal
+	</pre>
+
+	<pre class='descdef'>
+	Name: subscript-position-override
+	Value: auto | from-font | <<percentage>>
+	For: @font-face
+	Initial: normal
+	</pre>
+
+	<pre class='descdef'>
+	Name: superscript-size-override
+	Value: auto | from-font | <<percentage [0,Infinity]>>
+	For: @font-face
+	Initial: normal
+	</pre>
 
+	<pre class='descdef'>
+	Name: subscript-size-override
+	Value: auto | from-font | <<percentage [0,Infinity]>>
+	For: @font-face
+	Initial: normal
+	</pre>
+
+	The 'superscript-position-override', 'subscript-position-override',
+	'superscript-size-override', and 'subscript-size-override' descriptors
+	specify the superscript offset, subscript offset, superscript size, and subscript size
+	metrics of the font, respectively,
+	which are used to synthesize glyphs when required by 'font-variant-position'.
+
+	<dl dfn-for="superscript-position-override!!descriptor,subscript-position-override!!descriptor,superscript-size-override!!descriptor,subscript-size-override!!descriptor" dfn-type=value>
+		<dt><dfn>auto</dfn>
+		<dd>
+			The UA determines what metrics value to use,
+			whether derived from the font or from some heuristic.
 
+		<dt><dfn>from-font</dfn>
+		<dd>
+			The corresponding metric in the font data is used, if any.
+			(If the metric is missing, same as ''superscript-position-override/auto''.)
+
+		<dt><dfn><<percentage>></dfn>
+		<dd>
+			The corresponding metric is replaced by
+			the given percentage multiplied by the used font size.
+	</dl>
 
 <h2 id="font-rend-props">
 Font Feature Properties</h2>