[css-inline] Spec out some metric synthesis rules.

Author: fantasai

css-inline/Overview.bs

@@ -1090,28 +1090,105 @@ Interaction with floats</h4>
 	Issue: Is this the behavior we want? What if the float is on the same line as the initial letter?
 
 <h2 class="no-num" id="baseline-synthesis">
-Appendix A: Synthesizing Baselines</h2>
+Appendix A: Synthesizing Alignment Metrics</h2>
 
 	<h3 class="no-num" id="baseline-synthesis-fonts">
-	A.1: Synthesizing Baselines for Text</h3>
+	A.1: Synthesizing Baselines (and Other Font Metrics) for Text</h3>
 
-	Some fonts may not contain the baseline information necessary to align properly as described above.
-	User agents may follow these steps in the absence of a required metric:
+	Some fonts might not contain the metrics information necessary
+	to align text properly as described in this module.
+	User agents may use the following strategies
+	in the absence of a required metric:
 
-	<ol>
-
-		<li>Measure the font.
-			<div class="issue">Describe in more detail.
-			Should we document which characters in which scripts are useful,
-			such as lowercase "o" to <a href="https://drafts.csswg.org/css-values/#ex">measure x-height</a> or the middle of the minus sign for the mathematical baseline?</div>
+	<dl>
 
-		<li>Use a heuristic for the script.
+		<dt>Measure the font
+		<dd>
+			Metrics may be derived from the glyph shapes.
+			For example,
+			<ol>
+				<li>
+					The center of the minus sign (U+2212)
+					can be taken as the mathematical baseline.
+
+				<li>
+					The amount by which the lowercase “o”
+					descends below the alphabetic baseline
+					can be subtracted from its highest point
+					to <a href="https://drafts.csswg.org/css-values/#ex">measure the x-height</a>.
+
+				<li>
+					The amount by which the uppercase “O”
+					descends below the alphabetic baseline
+					can be subtracted from its highest point
+					to measure the cap-height.
+
+				<li>
+					The top edge of the below ideograph (U+4E05 “丅”)
+					and the bottom edge of the above ideograph (U+4E04 “丄”)
+					can be taken as the ideographic character face edges.
+					In the horizontal dimension,
+					the left and right edges of the nation ideograph (U+56FD “国”)
+					or solid ideograph (U+56FA “固”)
+					can be used.
+					In all cases, the these edges should be measured
+					at their intersection with the central baseline in that axis.
+
+					Issue: Is 国 or 固 more common?
+
+				<li>
+					The top edge of the center of the Hebrew maqaf (U+05B3 “־”)
+					can be taken as the Hebrew hanging baseline.
+
+				<li>
+					The top edge of the center of the letter Ka
+					can be taken as the hanging baseline.
+					Which Ka is used should depend on the <a>content language</a>:
+
+					<table class="data">
+						<thead>
+							<tr>
+								<th>Language
+								<th>Script
+								<th>Letter
+						<tbody>
+							<tr>
+								<td>
+								<td>Devanagari
+								<td>क U+0915 KA
+							<tr>
+								<td>
+								<td>Bengali
+								<td>ক U+0995
+							<tr>
+								<td>
+								<td>Gurmukhi
+								<td>ਕ U+0A15
+							<tr>
+								<td>
+								<td>Tibetan
+								<td>ཀ U+0F40
+					</table>
+
+					Issue: Pick a default.
+
+				<li>Issue: Add more notes here?
+			</ol>
+
+			Issue: Somebody sanity-check these heuristics please.
+
+		<dt>Use a heuristic for the script
+		<dd>
+			Issue: What does this mean?
 
-		<li>Use fallback values:
+		<dt>Use fallback values
+		<dd>
+			The following fallback values
 			<ul>
 				<li>x-height: .5em;
 				<li>cap-height: .66em;
 				<li>hanging baseline: .6em;
+				<li>Hebrew hanging baseline: ???
 			</ul>
 	</ol>
 

css-text/Overview.bs

@@ -282,7 +282,7 @@ Languages and Typesetting</h4>
     Authors should language-tag their content accurately for the best typographic behavior.
   </strong>
 
-  <p>The <dfn>content language</dfn> of an element is the (human) language
+  <p>The <dfn export>content language</dfn> of an element is the (human) language
     the element is declared to be in, according to the rules of the
     <a href="https://www.w3.org/TR/CSS21/conform.html#doclanguage">document language</a>.
     For example, the rules for determining the <a>content language</a> of an HTML