[css-ruby] Add ruby-position:alternate, and make it the initial value

See w3c#5971

Author: frivoal

css-ruby-1/Overview.bs

@@ -1285,8 +1285,8 @@ Ruby Positioning: the 'ruby-position' property</h3>
 
 	<pre class="propdef">
 	Name: ruby-position
-	Value: over | under | inter-character
-	Initial: over
+	Value: [ alternate || [ over | under ] ] | inter-character
+	Initial: alternate
 	Applies to: ruby annotation containers
 	Inherited: yes
 	Computed value: specified keyword
@@ -1296,9 +1296,26 @@ Ruby Positioning: the 'ruby-position' property</h3>
 	This property controls position of the [=ruby annotation=] with respect to its base.
 	Values have the following meanings:
 
-	Issue(5971): Roland Steiner has requested the addition of an auto value as default. See <a href="http://www.w3.org/Search/Mail/Public/advanced_search?keywords=&amp;hdr-1-name=subject&amp;hdr-1-query=ruby-position%3A+undesirable+default+value+%27before%27+for+complex+ruby&amp;hdr-2-name=from&amp;hdr-2-query=&amp;hdr-3-name=message-id&amp;hdr-3-query=&amp;period_month=&amp;period_year=&amp;index-grp=Public__FULL&amp;index-type=t&amp;type-index=www-style&amp;resultsperpage=20&amp;sortby=date">this thread</a> and <a href="http://www.w3.org/Search/Mail/Public/advanced_search?keywords=&amp;hdr-1-name=subject&amp;hdr-1-query=Styling+of+complex+Ruby&amp;hdr-2-name=from&amp;hdr-2-query=&amp;hdr-3-name=message-id&amp;hdr-3-query=&amp;period_month=&amp;period_year=&amp;index-grp=Public__FULL&amp;index-type=t&amp;type-index=public-i18n-core&amp;resultsperpage=20&amp;sortby=date">this one</a>. Current proposal is to add an optional ''alternate?'' keyword.
-
 	<dl dfn-for=ruby-position dfn-type=value>
+		<dt><dfn>alternate</dfn>
+		<dd>
+			Different [=levels=] of annotations alternate between ''over'' and ''under''.
+
+			If the [=annotation container=] is the first [=level=] of annotation in its [=ruby segment=],
+			or if all prior [=levels=] are [=inter-character=],
+			then ''alternate'', either on its own or in combination with ''over'',
+			behaves the same as ''over'',
+			while ''alternate'' in combination with ''under''
+			behaves the same as ''under''.
+
+			Otherwise,
+			if the previous level of [=interlinear=] annotation is ''over'',
+			''alternate''  behaves like ''under'',
+			and vice versa.
+			(In this case, whether ''alternate'' is specified alone
+			or in combination with ''over'' or ''under''
+			makes no difference.)
+
 		<dt><dfn>over</dfn>
 		<dd>
 			The ruby annotation appears [=line-over=] the base.