[css-text-3] Undefine where letter-spacing is applied. #1518

Author: fantasai

css-text-3/Overview.bs

@@ -4463,7 +4463,7 @@ Expanding and Compressing Text</h4>
     it is applied under the same rules as for 'word-spacing'.
     Similarly, when space is distributed to an <a>justification opportunity</a> between
     two <a>typographic character units</a>,
-    it is applied under the same rules as for 'letter-spacing'.
+    should be applied under the same rules as for 'letter-spacing'.
 
 
   <p>A justification algorithm may divide <a>justification opportunities</a> into different priority levels.
@@ -4926,7 +4926,8 @@ Word Spacing: the 'word-spacing' property</h3>
         letter-spacing/letter-spacing-204.html
       </wpt>
 
-    <p>Letter-spacing must not be applied at the beginning or at the end of a line.
+    <p>Letter-spacing must not be applied at the beginning of a line.
+    Whether letter-spacing is applied at the end of a line is undefined in this level.
 
     <wpt>
       letter-spacing/letter-spacing-end-of-line-001.html
@@ -4937,7 +4938,7 @@ Word Spacing: the 'word-spacing' property</h3>
     </wpt>
 
   <div class="example">
-    <p>Because letter-spacing is not applied at the beginning or end of a line,
+    <p>When letter-spacing is not applied at the beginning or end of a line,
     text always fits flush with the edge of the block.
     <pre>
       p    { letter-spacing: 1em; }
@@ -4947,7 +4948,8 @@ Word Spacing: the 'word-spacing' property</h3>
     <p class="ls-ex good ls-fixed-width color-box" style="text-align: left">a&#x3000;b&#x3000;c</p>
     <p class="ls-ex good ls-fixed-width color-box" style="text-align: right">a&#x3000;b&#x3000;c</p>
 
-    <p>UAs therefore must not append letter spacing to the right or trailing edge of a line:</p>
+    <p>UAs therefore <em>really should not</em> [[RFC6919]]
+    append letter spacing to the right or trailing edge of a line:
 
     <p class="ls-ex bad ls-fixed-width color-box" style="text-align: right">a&#x3000;b&#x3000;c&#x3000;</p>
   </div>
@@ -4957,6 +4959,12 @@ Word Spacing: the 'word-spacing' property</h3>
       the total letter spacing between two adjacent <a>typographic character units</a> (after bidi reordering)
       is specified by and rendered within
       the innermost element that <em>contains</em> the boundary between the two <a>typographic character units</a>.
+      However, the UA may instead attach letter-spacing at element boundaries
+      to one or the other <a>typographic character unit</a>
+      using the letter-spacing value pertaining to its containing element.
+
+      Note: This secondary behavior is permitted in this level
+      due to Web-compat concerns.
 
       <wpt>
         letter-spacing/letter-spacing-bidi-002.html
@@ -4970,45 +4978,45 @@ Word Spacing: the 'word-spacing' property</h3>
       </wpt>
 
   <div class="example">
-    <p>A given value of 'letter-spacing' only affects the spacing
-    between characters completely contained within the element for which it is specified:
+    <p>An inline box is expected to only include
+    letter spacing between characters completely contained within that element,
+    thus excluding letter spacing on the right or trailing edge of the element:
 
     <pre>
       p    { letter-spacing: 1em; }
-      span { letter-spacing: 2em; }
 
       &lt;p>a&lt;span>bb&lt;/span>c&lt;/p></pre>
 
-    <p class="ls-ex">a&#x3000;<span class="color-box">b&#x3000;&#x3000;b</span>&#x3000;c</p>
+    <p class="ls-ex good">a&#x3000;<span class="color-box">b&#x3000;b</span>&#x3000;c</p>
 
-    <p>This also means that applying 'letter-spacing' to
-    an element containing only a single character
-    has no effect on the rendered result:
+    <p class="ls-ex bad">a&#x3000;<span class="color-box">b&#x3000;b&#x3000;</span>c</p>
+
+    <p>Consequently a given value of 'letter-spacing' is expected
+    to only affect the spacing between characters
+    completely contained within the element for which it is specified:
 
     <pre>
       p    { letter-spacing: 1em; }
       span { letter-spacing: 2em; }
 
-      &lt;p>a&lt;span>b&lt;/span>c&lt;/p></pre>
+      &lt;p>a&lt;span>bb&lt;/span>c&lt;/p></pre>
 
-    <p class="ls-ex">a&#x3000;<span class="color-box">b</span>&#X3000;c</p>
+    <p class="ls-ex">a&#x3000;<span class="color-box">b&#x3000;&#x3000;b</span>&#x3000;c</p>
 
-    <p>An inline box only includes
-    letter spacing between characters completely contained within that element:
+    <p>This further implies that applying 'letter-spacing' to
+    an element containing only a single character
+    has no effect on the rendered result:
 
     <pre>
       p    { letter-spacing: 1em; }
+      span { letter-spacing: 2em; }
 
-      &lt;p>a&lt;span>bb&lt;/span>c&lt;/p></pre>
-
-    <p class="ls-ex good">a&#x3000;<span class="color-box">b&#x3000;b</span>&#x3000;c</p>
-
-    <p>It is incorrect to include the letter spacing on the right or trailing edge of the element:
+      &lt;p>a&lt;span>b&lt;/span>c&lt;/p></pre>
 
-    <p class="ls-ex bad">a&#x3000;<span class="color-box">b&#x3000;b&#x3000;</span>c</p>
+    <p class="ls-ex">a&#x3000;<span class="color-box">b</span>&#X3000;c</p>
 
-    <p>Letter spacing is inserted <strong>after</strong> RTL reordering,
-    so the letter spacing applied to the inner span below has no effect,
+    <p>Since letter spacing is inserted <strong>after</strong> RTL reordering,
+    the letter spacing applied to the inner span below likewise has no effect,
     since after reordering the "c" doesn't end up next to "&#x5d0;":
     <pre>
       p    { letter-spacing: 1em; }