[css-text][css-writing-modes] Consolidate definition of character.

Author: fantasai

css-text/Overview.bs

@@ -152,7 +152,7 @@ Terminology</h3>
 <h4 id="characters">
 Characters and Letters</h4>
 
-  <p>The basic unit of typesetting is the <dfn>character</dfn>.
+  <p>The basic unit of typesetting is the <dfn export>character</dfn>.
   However, because writing systems are not always as simple as the basic English alphabet,
   what a <a>character</a> actually is depends on the context in which the term is used.
   For example, in Hangul (the Korean writing system),
@@ -178,6 +178,8 @@ Characters and Letters</h4>
   <p>In turn, a single encoding <a>character</a> can be represented in the data stream as one or more bytes;
   and in programming environments one byte is sometimes also called a <a>character</a>.
 
+  <p>Therefore the term <a>character</a> is fairly ambiguous where technical precision is required.
+
   <p>For text layout, we will refer to the <dfn export lt="typographic character unit|typographic character">typographic character unit</dfn>
   as the basic unit of text.
   Even within the realm of text layout,
@@ -202,41 +204,53 @@ Characters and Letters</h4>
   as the basis for its <a>typographic character unit</a>.
   However, the UA should tailor the definitions
   as required by typographic tradition
-  since the default rules are not always appropriate or ideal,
+  since the default rules are not always appropriate or ideal--
   and is expected to tailor them differently
   depending on the operation as needed.
 
-<!--
   <p class="note">
-  The rules for such tailorings are out of scope for CSS,
+  The rules for such tailorings are out of scope for CSS.
+<!--
   however W3C currently maintains a wiki page
   where some known tailorings are collected.
 -->
 
   <div class="example">
-    <p>For example,
-    in some scripts such as Myanmar or Devanagari,
-    the <a>typographic character unit</a> for both justification and line-breaking
-    is an entire syllable,
-    which can include more than one [[!UAX29]] <a>grapheme cluster</a>.
-
-    <p>In other scripts such as Thai or Lao,
-    even though for line-breaking the <a>typographic character</a>
-    matches Unicode’s default <a>grapheme clusters</a>,
-    for letter-spacing the relevant unit
-    is <em>less</em> than a [[!UAX29]] <a>grapheme cluster</a>,
-    and may require decomposition or other substitutions
-    before spacing can be inserted.
-
-    <p>For instance,
-    to properly letter-space the Thai word คำ (U+0E04 + U+0E33),
-    the U+0E33 needs to be decomposed into U+0E4D + U+0E32,
-    and then the extra letter-space inserted before the U+0E32: คํ า.
-
-    <p>A slightly more complex example is น้ำ (U+0E19 + U+0E49 + U+0E33).
-     In this case, normal Thai shaping will first decompose the U+0E33 into U+0E4D + U+0E32
-     and then swap the U+0E4D with the U+0E49, giving U+0E19 + U+0E4D + U+0E49 + U+0E32.
-     As before the extra letter-space is then inserted before the U+0E32: นํ้ า.
+    The following are some examples of <a>typographic character unit</a> tailorings
+    required by standard typesetting practice:
+    
+    <ul>
+      <li>
+        <p>In some scripts such as Myanmar or Devanagari,
+        the <a>typographic character unit</a> for both justification and line-breaking
+        is an entire syllable,
+        which can include more than one [[!UAX29]] <a>grapheme cluster</a>.
+
+      <li>
+        <p>In other scripts such as Thai or Lao,
+        even though for line-breaking the <a>typographic character</a>
+        matches Unicode’s default <a>grapheme clusters</a>,
+        for letter-spacing the relevant unit
+        is <em>less</em> than a [[!UAX29]] <a>grapheme cluster</a>,
+        and may require decomposition or other substitutions
+        before spacing can be inserted.
+
+        <p>For instance,
+        to properly letter-space the Thai word คำ (U+0E04 + U+0E33),
+        the U+0E33 needs to be decomposed into U+0E4D + U+0E32,
+        and then the extra letter-space inserted before the U+0E32: คํ า.
+
+        <p>A slightly more complex example is น้ำ (U+0E19 + U+0E49 + U+0E33).
+        In this case, normal Thai shaping will first decompose the U+0E33 into U+0E4D + U+0E32
+        and then swap the U+0E4D with the U+0E49, giving U+0E19 + U+0E4D + U+0E49 + U+0E32.
+        As before the extra letter-space is then inserted before the U+0E32: นํ้ า.
+
+      <li>
+        <p>Vertical typesetting [[!CSS3-WRITING-MODES]] can also require tailoring.
+        For example, when typesetting ''text-orientation/upright'' text,
+        Tibetan tsek and shad marks are kept with the preceding grapheme cluster,
+        rather than treated as an independent <a>typographic character unit</a>.
+    </ul>
   </div>
 
   <p>A <dfn export>typographic letter unit</dfn> or <dfn>letter</dfn> for the purpose of this specification
@@ -2671,8 +2685,8 @@ Appendix C: Scripts and Spacing</h2>
 <h2 id="character-properties" class="no-num">Appendix D.
 Characters and Properties</h2>
 
-  <p>Unicode defines three codepoint-level properties that are referenced
-    in CSS Text:
+  <p>Unicode defines four codepoint-level properties that are referenced
+    in CSS typesetting:
   <dl export>
     <dt><dfn lt="Unicode East Asian Width|East Asian Width property"><a href="http://www.unicode.org/reports/tr11/#Definitions">East Asian width property</a></dfn>
     <dd>Defined in [[!UAX11]] and given as the <code>East_Asian_Width</code> property
@@ -2684,6 +2698,9 @@ Characters and Properties</h2>
     <dd>Defined in [[!UAX24]] and given as the <code>Script</code> property
       in the Unicode Character Database [[!UAX44]].
       (UAs must include any ScriptExtensions.txt assignments in this mapping.)
+    <dt><a href="http://www.unicode.org/reports/tr50/">Vertical Orientation</a>
+    <dd>Defined in [[!UTR50]] as the Vertical_Orientation property
+      and given in the UTR50 data file.
   </dl>
 
   <p>Unicode defines properties for individual codepoints, but sometimes