[css-text] Introduce a text-wrap-style value focused on orphans (w3c#10837)

See w3c#3473 (comment)

Co-authored-by: fantasai <fantasai.bugs@inkedblade.net>

Author: frivoal

css-text-4/Overview.bs

@@ -5501,7 +5501,7 @@ Selecting How to Wrap: the 'text-wrap-style' property</h3>
 
 	<pre class="propdef">
 	Name: text-wrap-style
-	Value: auto | balance | stable | pretty
+	Value: auto | balance | stable | pretty | avoid-orphans
 	Initial: auto
 	Applies to: [=block containers=] hat establish an [=inline formatting context=]
 	Inherited: yes
@@ -5612,9 +5612,89 @@ Selecting How to Wrap: the 'text-wrap-style' property</h3>
 		<dt><dfn>pretty</dfn>
 		<dd>
 			Specifies the UA <em>should</em> bias for better layout over speed,
-			and is expected to consider multiple lines,
+			and is expected to consider multiple lines
 			when making break decisions.
-			Otherwise equivalent to ''text-wrap-style/auto'',
+			Otherwise equivalent to ''text-wrap-style/auto''.
+
+			The user agent may among other things
+			attempt to avoid excessively short last lines,
+			similarly to ''avoid-orphans'',
+			but it should also improve the layout in additional ways.
+			The precise set of improvements is user agent dependent,
+			and may include things such as:
+			reducing the variation in length between lines;
+			avoiding <a href="https://en.wikipedia.org/wiki/River_(typography)">typographic rivers</a>;
+			prioritizing different classes of [=soft wrap opportunities=], [=hyphenation opportunities=], or [=justification opportunities=];
+			avoiding hyphenation on too many consecutive lines…
+
+			<div class=advisement>
+				The necessary computations may be expensive,
+				especially when applied to large amounts of text.
+				Authors are encouraged to assess the impact on performance
+				when using ''text-wrap-style: pretty'',
+				and possibly use it selectively where it matters most.
+			</div>
+
+		<dt><dfn>avoid-orphans</dfn>
+		<dd>
+			Specifies the UA <em>should</em> avoid excessively short last lines,
+			and is expected to consider more than one line
+			when making break decisions
+			(for example, to avoid "fixing" an orphan
+			by making the previous line too short).
+			The user agent may decide against improving the last line
+			when it would make some prior line(s) substantially unbalanced.
+			The user agent should not attempt to improve other aspects of the layout
+			beyond what ''text-wrap-style/auto''
+			if those improvements come at a significant performance cost.
+
+			Note: In western typography, an “orphan” can refer to a single word on the last line of a paragraph.
+			However, not all languages or writing systems wrap lines at word boundaries,
+			and even in languages that do,
+			a few very short words on the last line may still be undesirable,
+			especially if the available inline space is long.
+			This specification does not therefore strictly define
+			lines to be too short only when they are composed a single word,
+			and lets the user agent be the judge of what is excessively short.
+
+			Note: See also the 'orphans' property for control over fragmentation
+			and the other typographic meaning of “orphans”.
+
+			<div class=example>
+				For instance, with the given line length, the following test would be wrapped with a single word on the last line,
+				which could be judged too short.
+				<pre lang=grc style="border: solid lightgray 8px; border-bottom: none; border-top: none; width: 45ch;">
+ἄνδρα μοι ἔννεπε, μοῦσα, πολύτροπον, ὃς μάλα
+πολλὰ πλάγχθη, ἐπεὶ Τροίης ἱερὸν πτολίεθρον
+ἔπερσεν.
+				</pre>
+				Applying ''text-wrap-style: avoid-orphans'' could yield something like this instead,
+				which is arguably more pleasing:
+				<pre lang=grc style="border: solid lightgray 8px; border-bottom: none; border-top: none; width: 45ch;">
+ἄνδρα μοι ἔννεπε, μοῦσα, πολύτροπον, ὃς
+μάλα πολλὰ πλάγχθη, ἐπεὶ Τροίης ἱερὸν
+πτολίεθρον ἔπερσεν.
+				</pre>
+			</div>
+			<div class=example>
+				However, the following fragment is more vexing:
+				<pre style="border: solid lightgray 8px; border-bottom: none; border-top: none; width: 17ch;">
+Circumnavigating
+the Mississippi
+river
+				</pre>
+				An attempt to make the last line less short by wrapping the previous line earlier would result in the following:
+				<pre style="border: solid lightgray 8px; border-bottom: none; border-top: none; width: 17ch;">
+Circumnavigating
+the
+Mississippi river
+				</pre>
+				While the last line would indeed no longer be short,
+				the penultimate line would be unsightly.
+				In such cases, user agents are expected
+				to prefer the first rendering
+				despite ''text-wrap-style: avoid-orphans'' being set.
+			</div>
 	</dl>
 
 	<!-- add a sample prioritization algorithm -->
@@ -5626,10 +5706,12 @@ Selecting How to Wrap: the 'text-wrap-style' property</h3>
 	UAs can experiment with better line breaking algorithms
 	with this default value,
 	but as optimal results often take more time,
-	''text-wrap-style/pretty'' is offered as an opt-in
+	''text-wrap-style/pretty'' and ''avoid-orphans'' are offered as opt-ins
 	to take more time for better results.
-	The ''text-wrap-style/pretty'' value is intended for body text,
-	where the last line is expected to be a bit shorter than the average line;
+	Both values are intended for body text,
+	where the last line is expected to be a bit shorter than the average line,
+	but ''avoid-orphans'' aims to avoid excessively short last lines while staying performant,
+	while ''pretty'' can sacrifice more speed in pursuit of even better layout;
 	the ''text-wrap-style/balance'' value is intended for titles and captions,
 	where equal-length lines of text tend to be preferred;
 	and the ''text-wrap-style/stable'' is intended for sections that are,