[css-inline-3] Define em-over and em-under baselines for Canvas API. #5312

Author: fantasai

css-inline-3/Overview.bs

@@ -428,6 +428,18 @@ Baselines and 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="em-over baseline" local-lt="em-over">em-over</dfn>
+		<dd>
+			Corresponds to a conceptual [=ascent=] normalized
+			to ensure 1em between [=em-over=] and [=em-under=].
+			See [[#baseline-synthesis-em]].
+
+		<dt><dfn lt="em-under baseline" local-lt="em-under">em-under</dfn>
+		<dd>
+			Corresponds to a conceptual [=descent=] normalized
+			to ensure 1em between [=em-over=] and [=em-under=].
+			See [[#baseline-synthesis-em]].
 	</dl>
 
 	Note: These metrics are optical design metrics,
@@ -2655,8 +2667,37 @@ Interaction with Fragmentation (Pagination)</h4>
 <h2 class="no-num" id="baseline-synthesis">
 Appendix A: Synthesizing Alignment Metrics</h2>
 
-	<h3 class="no-num" id="baseline-synthesis-fonts">
-	A.1: Synthesizing Baselines (and Other Font Metrics) for Text</h3>
+<h3 class="no-num" id="baseline-synthesis-em">
+A.1: Calculating [=Em-over=] and [=Em-under=]</h3>
+
+	Note: The [=em-over=] and [=em-under=] baselines are not used by CSS.
+	Their definitions are included in this module
+	for consistency with the other metrics used by
+	<a href="https://html.spec.whatwg.org/multipage/canvas.html#2dcontext">Canvas 2D API</a>.
+
+	The [=em-over=] and [=em-under=] metrics are calculated as follows:
+
+	* If any one of
+		[=central=], [=ideographic-over=], or [=ideographic-under=]
+		is defined by the font,
+		then [=em-over=] is 0.5em over the [=central=] baseline,
+		and [=em-under=] is 0.5em under it,
+		with the [=central=] baseline derived from the others
+		if missing or undefined
+		(see below).
+
+	* Otherwise, the [=ascent=] and [=descent=]
+		are both proportionally augmented or reduced
+		to add up to exactly 1em,
+		and these normalized metrics are taken
+		as the [=em-over=] and [=em-under=] metrics, respectively.
+
+	Note: This calculation ensures that [=em-over=] and [=em-under=]
+	are always exactly 1em apart
+	while trying to center the glyph outlines’ “center of gravity” between them.
+
+<h3 class="no-num" id="baseline-synthesis-fonts">
+A.2: Synthesizing Baselines (and Other Font Metrics) for Text</h3>
 
 	Some fonts might not contain the metrics information necessary
 	to align text properly as described in this module.
@@ -2681,7 +2722,7 @@ Appendix A: Synthesizing Alignment Metrics</h2>
 					The [=ideographic-over=] and [=ideographic-under=] baselines
 					are typically ''1em'' apart,
 					so if only one of the
-					[=ideographic-over=]/[=ideographic-under=]/[=ideograph-central=]
+					[=ideographic-over=]/[=ideographic-under=]/[=central=]
 					baselines are provided,
 					this relation can be used to calculate the other two.
 				<li>
@@ -2781,10 +2822,10 @@ Appendix A: Synthesizing Alignment Metrics</h2>
 				<li>cap-height: .66em;
 				<li>hanging baseline: .6em;
 			</ul>
-	</ol>
+	</dl>
 
 <h3 class="no-num" id="baseline-synthesis-box">
-A.2: Synthesizing Baselines for Atomic Inlines</h3>
+A.3: Synthesizing Baselines for Atomic Inlines</h3>
 
 	If an [=atomic inline=]
 	(such as an inline-block, inline-table, or [=replaced element=])