[css-text] Clarify that suppressed letter-spacing among Arabic chars is effectively zero letter-spacing (hence ligatures are applied).

--HG--
extra : rebase_source : 79c273d8fec6beb1de87a4cda13d533888168e93

Author: fantasai

css-text/Overview.bs

@@ -1985,31 +1985,33 @@ Word Spacing: the 'word-spacing' property</h3>
     <p class="ls-ex">a&emsp;b</span>&emsp;<span class="color-box">c</span>&emsp;<span class="color-box">&#x5d0;</span>&emsp;&#x5d1;&emsp;&#x5d2;</p>
   </div>
 
-    <p>Letter-spacing ignores invisible zero-width formatting characters
+    <p>Letter spacing ignores invisible zero-width formatting characters
       (such as those from the Unicode Cf category).
       Spacing must be added as if those characters did not exist in the document.
 
     <p class="example">For example, 'letter-spacing' applied to
       <code>A&amp;#x200B;B</code> is identical to <code>AB</code>,
       regardless of where any element boundaries might fall.
 
-    <p>When the effective letter-spacing between two characters is not zero
+    <p>When the effective spacing between two characters is not zero
       (due to either <a href="#text-justify">justification</a>
-      or non-zero computed 'letter-spacing'),
+      or a non-zero value of 'letter-spacing'),
       user agents should not apply optional ligatures.</p>
 
     <p class="example">
       For example, if the word “filial” is letter-spaced,
       an “fi” ligature should not be used
       as it will prevent even spacing of the text.
 
-    <p>If it is able, the UA <em>may</em> apply letter-spacing to <i>cursive scripts</i>
-    by translating the total spacing distributed to a run of such letters
+    <p>If it is able, the UA <em>may</em> apply letter spacing to <i>cursive scripts</i>
+    by translating the total extra space to be distributed to a run of such letters
     into some form of cursive elongation for that run.
     Otherwise, if the UA cannot expand text from a <i>cursive script</i>
     without breaking its cursive connections,
     it <em>must not</em> apply spacing
     between any pair of that script's <i>visually-perceived characters</i> at all.
+    (Both cases will result in an effective spacing of zero between such letters,
+    however the former will preserve the sense of stretching out the text.)
 
 <h2 id="edge-effects">
   Edge Effects</h2>

css-text/Overview.html

@@ -2122,31 +2122,33 @@ <h3 class="heading settled heading" data-level=8.2 id=letter-spacing-property><s
     <p class=ls-ex>a b <span class=color-box>c</span> <span class=color-box>א</span> ב ג</p>
   </div>
 
-    <p>Letter-spacing ignores invisible zero-width formatting characters
+    <p>Letter spacing ignores invisible zero-width formatting characters
       (such as those from the Unicode Cf category).
       Spacing must be added as if those characters did not exist in the document.
 
     <p class=example>For example, <a class=property data-link-type=propdesc href=#propdef-letter-spacing title=letter-spacing>letter-spacing</a> applied to
       <code>A&amp;#x200B;B</code> is identical to <code>AB</code>,
       regardless of where any element boundaries might fall.
 
-    <p>When the effective letter-spacing between two characters is not zero
+    <p>When the effective spacing between two characters is not zero
       (due to either <a href=#text-justify>justification</a>
-      or non-zero computed <a class=property data-link-type=propdesc href=#propdef-letter-spacing title=letter-spacing>letter-spacing</a>),
+      or a non-zero value of <a class=property data-link-type=propdesc href=#propdef-letter-spacing title=letter-spacing>letter-spacing</a>),
       user agents should not apply optional ligatures.</p>
 
     <p class=example>
       For example, if the word “filial” is letter-spaced,
       an “fi” ligature should not be used
       as it will prevent even spacing of the text.
 
-    <p>If it is able, the UA <em>may</em> apply letter-spacing to <a data-link-type=dfn href=#cursive-script title="cursive scripts">cursive scripts</a>
-    by translating the total spacing distributed to a run of such letters
+    <p>If it is able, the UA <em>may</em> apply letter spacing to <a data-link-type=dfn href=#cursive-script title="cursive scripts">cursive scripts</a>
+    by translating the total extra space to be distributed to a run of such letters
     into some form of cursive elongation for that run.
     Otherwise, if the UA cannot expand text from a <a data-link-type=dfn href=#cursive-script title="cursive script">cursive script</a>
     without breaking its cursive connections,
     it <em>must not</em> apply spacing
     between any pair of that script’s <a data-link-type=dfn href=#visually-perceived-character title="visually-perceived characters">visually-perceived characters</a> at all.
+    (Both cases will result in an effective spacing of zero between such letters,
+    however the former will preserve the sense of stretching out the text.)
 
 <h2 class="heading settled heading" data-level=9 id=edge-effects><span class=secno>9 </span><span class=content>
   Edge Effects</span><a class=self-link href=#edge-effects></a></h2>