[css-text-3] More precisely define interaction of letter-spacing and ligature font features. #2644

Author: fantasai

css-text-3/Overview.bs

@@ -1945,7 +1945,8 @@ Word Spacing: the 'word-spacing' property</h3>
   <p>This property specifies additional spacing (commonly called <dfn export>tracking</dfn>)
   between adjacent <a>typographic character units</a>.
   Letter-spacing is applied after
-  <a href="https://www.w3.org/TR/css-writing-modes-3/#text-direction">bidi reordering</a>
+  <a href="https://www.w3.org/TR/css-writing-modes/#text-direction">bidi reordering</a> [[CSS-WRITING-MODES-3]]
+  and <a href="https://www.w3.org/TR/css-fonts/#font-kerning-prop">kerning</a> [[CSS-FONTS-3]]
   and is in addition to any 'word-spacing'.
   Depending on the justification rules in effect,
   user agents may further increase or decrease the space between <a>typographic character units</a>
@@ -2057,9 +2058,13 @@ Word Spacing: the 'word-spacing' property</h3>
     <p>When the effective spacing between two characters is not zero
       (due to either <a href="#text-justify-property">justification</a>
       or a non-zero value of 'letter-spacing'),
-      user agents should not apply optional ligatures.
-      However, kerning should still be applied
-      (see 'font-kerning' [[CSS-FONTS-3]]).
+      user agents should not apply optional ligatures,
+      i.e. those that are not defined as required
+      for fundamentally correct glyph shaping.
+      However, ligatures and other font features
+      specified via the low-level 'font-feature-settings' property
+      take precedence over this rule.
+      See [[css-fonts-3#feature-precedence]].
 
     <div class="example">
       For example, if the word “filial” is letter-spaced,
@@ -2071,6 +2076,13 @@ Word Spacing: the 'word-spacing' property</h3>
       </figure>
     </div>
 
+    Note: In OpenType, required ligatures are expected
+    to be associated to the <code>rlig</code> feature.
+    All other ligatures are therefore considered optional.
+    In some cases, however, UA or platform heuristics
+    apply additional ligatures in order to handle broken fonts;
+    this specification does not define or override such exceptional handling.
+
 <h4 id="cursive-tracking">
 Cursive Scripts</h4>
 
@@ -3128,7 +3140,7 @@ Acknowledgements</h2>
   Laurie Anna Edlund, Ben Errez, Yaniv Feinberg, Arye Gittelman, Ian
   Hickson, Martin Heijdra, Richard Ishida, Masayasu Ishikawa,
   Michael Jochimsen, Eric LeVine, Ambrose Li, Håkon Wium Lie, Chris Lilley,
-  Ken Lunde, Nat McCully, IM Mincheol, Shinyu Murakami, Paul Nelson,
+  Ken Lunde, Myles Maxfield, Nat McCully, IM Mincheol, Shinyu Murakami, Paul Nelson,
   Chris Pratley, Xidorn Quan, Marcin Sawicki,
   Arnold Schrijver, Rahul Sonnad, Alan Stearns, Michel Suignard, Takao Suzuki,
   Frank Tang, Chris Thrasher, Etan Wexler, Chris Wilson, Masafumi Yabe