[css-text-4] Add the text-group-align property

This is the first step towards solving w3c#1975

r=fantasai

Author: frivoal

css-text-4/Overview.bs

@@ -932,6 +932,67 @@ Character-based Alignment in a Table Column</h3>
 	and to the without-alignment-character intrinsic widths
 	of the other spanned columns.</p>
 
+<h3 id="text-group-align-property">
+Aligning a block of text within its container: the 'text-group-align' property</h3>
+
+	<pre class="propdef">
+	Name: text-group-align
+	Value: none | start | end | left | right | center
+	Initial: none
+	Applies to: <a>block containers</a>
+	Inherited: no
+	Percentages: N/A
+	Computed value: as specified
+	</pre>
+
+	This property aligns the contents of the line boxes as a group while maintaining their text alignment.
+
+	<dfn lt="group alignment | group-align | group-aligned">Group alignment</dfn> is performed by finding the line box with the shortest remaining space
+	and adding that amount of space as padding to one or both sides of the line box,
+	reducing the amount of space available for its contents;
+	<a href="#text-align-property">text alignment</a> is then applied to its contents within the remaining space.
+	All descendant [=in-flow=] line boxes within the same [=block formatting context=] are considered
+	both when searching for the shortest remaining space
+	and when adding the padding;
+	the contents of descendants that establish [=independent formatting contexts=] are skipped.
+
+	Issue: A variant of this property is inherited,
+	and applies on each block container individually,
+	only affecting the line boxes that are direct children of that block.
+	This is less useful, but probably easier to implement.
+
+	Issue: Somehow also moving the floats that originate in the same block container
+	by the same amount
+	would make things line up more nicely,
+	which would be especially valuable in CJK layout.
+	Exactly how that works, and how it interacts with intruding floats
+	from ancestor elements is left as an exercise for the reader.
+
+	Values have the following meanings:
+
+	<dl dfn-for=text-group-align dfn-type=value>
+		<dt><dfn>none</dfn>
+		<dd>Text alignment happens normally: [=group alignment=] is not performed.
+
+		<dt><dfn>start</dfn>
+		<dd>Inline-level content is [=group-aligned=] to the [=inline start=] side,
+		    by padding the [=inline end=] side of each line box.
+		<dt><dfn>end</dfn>
+		<dd>Inline-level content is [=group-aligned=] to the [=inline end=] side,
+		    by padding the [=inline start=] side of each line box.
+		<dt><dfn>left</dfn>
+		<dd>Inline-level content is [=group-aligned=] to the [=line-left=] side,
+		    by padding the [=line-right=] side of each line box.
+		<dt><dfn>right</dfn>
+		<dd>Inline-level content is [=group-aligned=] to the [=line-right=] side,
+		    by padding the [=line-left=] side of each line box.
+		<dt><dfn>center</dfn>
+		<dd>Inline-level content is [=group-aligned=] to the center,
+		    by padding both sides of each line box,
+		    half the spacing to each side.
+	</dl>
+
+
 
 
 <h2 id="spacing">