[css-ruby] Add ruby-overhang:spaces

Closes #5912

Author: frivoal

css-ruby-1/Overview.bs

@@ -24,6 +24,11 @@ Abstract: This module describes the rendering model and formatting controls rela
 <pre class=link-defaults>
 spec:css-box-4; type:dfn; text:content area
 spec:css-text-3; type:dfn; text:character
+spec:css-text-3; type:dfn; text:other space separators
+spec:css-text-4; type:dfn;
+	text:fullwidth opening punctuation
+	text:fullwidth closing punctuation
+	text:fullwidth middle dot punctuation
 spec:css-writing-modes-4; type:dfn; text:bidi isolation
 spec:html-ruby-extensions; type:element;
 	text:rp
@@ -1821,7 +1826,7 @@ Overhanging Ruby: the 'ruby-overhang' property</h3>
 
 	<pre class="propdef">
 	Name: ruby-overhang
-	Value: auto | none
+	Value: auto | spaces
 	Initial: auto
 	Applies to: ruby annotation containers
 	Inherited: yes
@@ -1844,10 +1849,24 @@ Overhanging Ruby: the 'ruby-overhang' property</h3>
 			Whether, how much, and under which conditions to overhang
 			are determined by the UA.
 
-		<dt><dfn>none</dfn>
+		<dt><dfn>spaces</dfn>
 		<dd>
-			A [=ruby annotation container=] is never allowed
-			to extend past the [=ruby annotation container=].
+			When a [=ruby annotation container=] is longer than
+			its corresponding [=ruby base container=],
+			the [=ruby annotation container=] must not
+			overlap adjacent boxes,
+			except over adjacent spaces.
+
+			For this purpose,
+			spaces are any of the following:
+			* [=preserved white space=]
+			* [=other space separators=]
+			* half of the [=advance measure=] of [=fullwidth opening punctuation=],
+				on the [=inline-start=] side of the character
+			* half of the [=advance measure=] of [=fullwidth closing punctuation=],
+				on the [=inline-end=] side of the character
+			* one quarter of the [=advance measure=] of [=fullwidth middle dot punctuation=],
+				on both the [=inline-start=] and [=inline-end=] sides of the character
 	</dl>
 
 	When [=ruby annotations=] are not allowed to overhang,
@@ -1900,6 +1919,11 @@ Overhanging Ruby: the 'ruby-overhang' property</h3>
 		for Japanese are described by [[JLREQ]].
 	</div>
 
+	Additionally, for compatibility reasons,
+	user agents must support an additional value of <dfn value for=ruby-overhang>none</dfn>
+	for the 'ruby-overhang' property,
+	as a [=legacy value alias=] of ''ruby-overhang/spaces''.
+
 <h3 id="line-edge">
 Line-edge Alignment</h3>