[css-inline-3] Add baseline-preference property, naming 'baseline-source' for now. #861 <https://lists.w3.org/Archives/Public/www-style/2019May/0011.html>

Author: fantasai

css-inline-3/Overview.bs

@@ -155,29 +155,58 @@ Transverse Box Alignment: the 'vertical-align' property</h3>
 
 	<pre class="propdef shorthand">
 	Name: vertical-align
-	Value: <<'baseline-shift'>> || <<'alignment-baseline'>>
+	Value: <<'baseline-source'>> || <<'baseline-shift'>> || <<'alignment-baseline'>>
 	Initial: baseline
 	Applies to: inline-level boxes
 	Inherited: no
 	Percentages: N/A
 	</pre>
 
-	<p>This shorthand property specifies how an inline-level box is aligned within the line.
+	<p>This <a>shorthand</a> property specifies
+	how an inline-level box is aligned within the line.
 	Values are the same as for its longhand properties, see below.
 
 	<p class="advisement">
-		Authors should use this property ('vertical-align') instead of its longhands.
+		Authors should use this property ('vertical-align') instead of its longhands
+		(unless it is specifically needed to cascade its longhands independently).
 
-	ISSUE: This property will gain ''first'' and ''last'' keywords,
-	like in the <a href="https://www.w3.org/TR/css-align-3/">box alignment properties</a>,
-	see [[css-align-3#baseline-values]].
-	The <a href="https://github.com/w3c/csswg-drafts/issues/861">open question</a>
-	is whether they should be added to 'alignment-baseline'
-	or a new <a>sub-property</a> should be created
-	to hold the ''first | last'' preference.
+<h4 id="baseline-source">
+Alignment Baseline Source: the 'baseline-source' longhand</h4>
+
+	<pre class="propdef">
+	Name: baseline-source
+	Value: auto | first | last
+	Initial: auto
+	Applies to: inline-level boxes
+	Inherited: no
+	Percentages: N/A
+	Computed value: specified keyword
+	Animation type: discrete
+	</pre>
+
+	ISSUE: This is a rough draft, not ready for implementation. Also might rename stuff.
+
+	When an inline-level box
+	has more than one possible source for baseline information
+	(such as for a multi-line inline block or inline flex container)
+	this property specifies whether the <a>first baseline set</a> or <a>last baseline set</a>
+	is preferred for alignment.
+	Values have the following meanings:
+
+	<dl dfn-for=alignment-baseline dfn-type=value>
+		<dt><dfn>auto</dfn>
+		<dd>Specifies <a>last-baseline alignment</a> for ''inline-block'',
+		<a>first-baseline alignment</a> for everything else.
+
+		<dt><dfn>first</dfn>
+		<dd>Specifies <a>first-baseline alignment</a>.
+
+		<dt><dfn>last</dfn>
+		<dd>Specifies <a>last-baseline alignment</a>.
+	</dl>
 
 <h4 id="alignment-baseline-property">
-Alignment Point: 'alignment-baseline' longhand</h4>
+Alignment Baseline Type: the 'alignment-baseline' longhand</h4>
 
 	<pre class="propdef">
 	Name: alignment-baseline