[css-text] More cleanup up of possible justification algorithms section. Add sentence imposing requirement for correctly handling complex scripts regardless of justification.

--HG--
extra : rebase_source : 2d9105c280dbb841d6b76e656ecaf81b1806344a

Author: fantasai

css-text/Overview.html

@@ -2284,17 +2284,13 @@ <h4 id=justify-cursive><span class=secno>7.3.4. </span> Cursive Scripts</h4>
 
   <h4 id=justify-alt><span class=secno>7.3.5. </span> Possible Algorithms</h4>
 
-  <p>The UA may enable or break optional ligatures or use other font features
-   such as alternate glyphs or glyph compression to help justify the text
-   under any method. This behavior is not controlled by this level of CSS.
-
-  <div class=note>
-   <p>The guidelines in this level of CSS do not describe a complete
-    justification algorithm. They are merely a minimum set of requirements
-    that a complete algorithm should meet. Limiting the set of requirements
-    gives UAs some latitude in choosing a justification algorithm that meets
-    their needs and desired balance of quality, speed, and complexity.
+  <p>The guidelines in this level of CSS do not describe a complete
+   justification algorithm. They are merely a minimum set of requirements
+   that a complete algorithm should meet. Limiting the set of requirements
+   gives UAs some latitude in choosing a justification algorithm that meets
+   their needs and desired balance of quality, speed, and complexity.
 
+  <div class=example>
    <p>For instance, a basic but fast ‘<code class=css>inter-word</code>’
     justification algorithm might use a simple greedy method for determining
     line breaks, then distribute leftover space. This algorithm could follow
@@ -2309,201 +2305,34 @@ <h4 id=justify-alt><span class=secno>7.3.5. </span> Possible Algorithms</h4>
     considerations. This algorithm could follow the guidelines by giving more
     weight to <a href="#word-separator"><i>word separators</i></a> than
     letter spacing.
-
-   <p>It's not defined whether expansion or compression is preferred, so a UA
-    may, for example, bias towards compression for CJK languages but towards
-    expansion for Western alphabetic languages.
   </div>
-  <!--
-    <p>The <i>expansion opportunity</i> priorities for values of 'text-justify'
-      are given in the table below. Since justification behavior
-      varies by writing system, <i>expansion opportunities</i> are organized by
-      <a href="#script-groups">script categories</a>.
-      An <i>expansion opportunity</i> exists between two <i>letters</i> at a
-      priority level when at least one of them belongs to a script category
-      at that level and the other does not belong to a higher priority level.
-      All scripts in the same priority level must be treated exactly the same.
-      Word separators (spaces) and other symbols and punctuation are treated
-      specially, see below.
-
-    <table class="data">
-      <caption>Prioritization of Expansion Points</caption>
-      <colgroup class="header"></colgroup>
-      <colgroup span=6></colgroup>
-      <thead>
-        <tr>
-          <td></td>
-          <th>''inter-word''</th>
-          <th>''inter-ideograph''</th>
-          <th>''distribute''</th>
-          <th>''inter-cluster''</th>
-          <th>''kashida''</th>
-          <th>''auto''</th>
-        </tr>
-      </thead>
-      <tbody>
-        <tr>
-          <th scope="row"><a href="#block-scripts">block</a></th>
-          <td>2</td>
-          <td><strong>1</strong></td>
-          <td><strong>1</strong></td>
-          <td>3</td>
-          <td>3</td>
-          <td><strong>2</strong>*</td>
-        </tr>
-
-        <tr>
-          <th scope="row"><a href="#clustered-scripts">clustered</a></th>
-          <td>2</td>
-          <td>2</td>
-          <td><strong>1</strong></td>
-          <td><strong>2</strong></td>
-          <td>3</td>
-          <td><strong>2</strong>*</td>
-        </tr>
-
-        <tr>
-          <th scope="row"><a href="#cursive-scripts">cursive</a></th>
-          <td>2</td>
-          <td>2</td>
-          <td>2</td>
-          <td>3</td>
-          <td><strong>1</strong></td>
-          <td>3*</td>
-        </tr>
-
-        <tr>
-          <th scope="row"><a href="#discrete-scripts">discrete</a></th>
-          <td>2</td>
-          <td>2</td>
-          <td><strong>1</strong></td>
-          <td>3</td>
-          <td>3</td>
-          <td>3*</td>
-        </tr>
-
-        <tr>
-          <th scope="row"><a href="#connected-scripts">connected</a></th>
-          <td>never</td>
-          <td>never</td>
-          <td>never</td>
-          <td>never</td>
-          <td>never</td>
-          <td>never</td>
-        </tr>
-      </tbody>
-
-      <tbody>
-        <tr>
-          <th scope="row">spaces</th>
-          <td><strong>1</strong></td>
-          <td><strong>1</strong></td>
-          <td><strong>1</strong></td>
-          <td><strong>1</strong></td>
-          <td>2</td>
-          <td><strong>1</strong>*</td>
-        </tr>
-
-        <tr>
-          <th scope="row">symbols</th>
-          <td>2</td>
-          <td><strong>1</strong></td>
-          <td><strong>1</strong></td>
-          <td><strong>2</strong></td>
-          <td>3</td>
-          <td>*</td>
-        </tr>
-      </tbody>
-    </table>
-
-    <p id="auto-justify"><small>* The ''auto'' column defined above is informative; it
-      suggests a prioritization that presents a universal compromise among
-      justification methods.</small></p>
-    <p id="justify-spaces">The <dfn title="spaces-category">spaces</dfn> category represents
-      <i>expansion opportunities</i> at <a href="#word-separator">word separators</a>.
-      (See <a href="#word-spacing">'word-spacing'</a>.)
-      Except when 'text-justify' is ''distribute'', the UA may treat
-      spaces differently than other <i>expansion opportunities</i> in the same priority,
-      but must not change their priority with respect to <i>expansion opportunities</i>
-      in other priority levels. For example, in Japanese ''inter-ideograph''
-      justification (which treats CJK characters at a higher priority than
-      Latin characters), word spaces traditionally have a higher priority
-      than inter-CJK spacing, and the UA may split the 1st-priority level
-      to implement that. However the UA is not allowed to drop either spaces
-      or CJK characters to the same priority as Latin characters.
-    <p id="justify-symbols">The <dfn id="punctuation-symbols">symbols</dfn>
-      category represents the <i>expansion opportunity</i> existing at or between
-      any pair of characters from the Unicode Symbols (S*) and Punctuation (P*)
-      classes.
-      The default justification priority of these <i>expansion opportunities</i> is given above.
--->
-  <!--
-    <p id="justify-cursive">For justification of <i>cursive scripts</i>,
-      words may be expanded through kashida elongation or other cursive
-      expansion processes. Kashida may be applied in discrete units or
-      continuously, and the prioritization of kashida opportunities is UA-dependent:
-      for example, the UA may apply more at the end of the line. The
-      UA should not apply kashida to fonts for which it is inappropriate.
-      It may instead rely on other justification methods that lengthen
-      or shorten Arabic segments (e.g. by substituting in swash forms or
-      optional ligatures). Because elongation rules depend on the typeface
-      style, the UA should rely on on the font whenever possible rather
-      than inserting kashida based on a font-independent style rule. The UA
-      should limit elongation so that, e.g. in multi-script lines a short
-      stretch of Arabic will not be forced to soak up too much of the
-      extra space by itself. If the UA does not support cursive elongation,
-      then, as with connected scripts, no <i>expansion opportunities</i> exist between
-      characters of these scripts.
--->
 
   <div class=example>
-   <p>3.8 Line Adjustment in <a href="#JLREQ"
+   <p>A UA can also tailor its justification rules by language, to produce
+    results more closely aligned to the typography of that language. For
+    example, it's not defined whether expansion or compression is preferred,
+    so a UA may, for example, bias towards compression for CJK languages but
+    towards expansion for Western alphabetic languages.
+
+   <p>As another example, 3.8 Line Adjustment in <a href="#JLREQ"
     rel=biblioentry>[JLREQ]<!--{{JLREQ}}--></a> gives an example of a set of
-    rules for how a text formatter can justify Japanese text. It describes
-    rules for cases where the ‘<a href="#text-justify0"><code
+    rules for how a text formatter can justify Japanese text. A UA could use
+    this algorithm when the ‘<a href="#text-justify0"><code
     class=property>text-justify</code></a>’ property is ‘<a
-    href="#auto1"><code class=css>auto</code></a>’.</p>
-   <!--
-      <p>It produces an effect similar to cases where
-        the computed value of 'text-spacing' property does not specify
-        ''trim-end'' or ''space-end''.
-        If the UA wants to prohibit this behavior,
-        rule b. of 3.8.3 should be omitted.</p>
--->
+    href="#auto1"><code class=css>auto</code></a>’. However, since the
+    rules described in the document specifically target Japanese, they may
+    produce non-optimal results when used to justify other languages such as
+    English. The UA could adapt the rules to accommodate other scripts by,
+    for instance, omitting the rule to compress half-width spaces (rule a. of
+    3.8.3). Or it could keep the rule, but only enable it when the content
+    language is known to be Japanese.
+  </div>
 
-   <p>Note that the rules described in the document specifically target
-    Japanese. Therefore they may produce non-optimal results when used to
-    justify other languages such as English. To make the rules more
-    applicable to other scripts, the UA could, for instance, omit the rule to
-    compress half-width spaces (rule a. of 3.8.3).</p>
-   <!--
-      <p>JIS X-4051 [[JIS4051]] defines how a text formatter can justify Japanese text.
-        Here is one example of the interpretation of JIS X-4051 with slight modification.</p>
-        <ol>
-          <li>If no justification is necessary, neither compression nor expansion occur.</li>
-          <li>If justification is necessary,
-            take the first soft wrap opportunity beyond the end of line
-            and apply the following rules (in order) to compress until it fits.
-            <ol style="list-style-type:lower-alpha">
-              <li>Compress space characters up to the minimum value specified by 'word-spacing' property, or up to 1/4em.</li>
-              <li>Compress <a href="#fullwidth-middle-dot-punctuation">fullwidth middle dot punctuations</a>
-                and <a href="#fullwidth-colon-punctuation">fullwidth colon punctuations</a>
-                up to 1/2em, by trimming the same amount of spaces from both sides of the characters.</li>
-              <li>Compress the left side of <a href="#fullwidth-opening-punctuation">fullwidth opening punctuations</a>
-                and the right side of <a href="#fullwidth-closing-punctuation">fullwidth closing punctuations</a>
-                up to 1/2em.</li>
-              <li>Compress spaces created by 'text-spacing' property up to 1/8em.</li>
-            </ol></li>
-          <li>If the compression fails to fit the line,
-            take the last soft wrap opportunity before the end of line,
-            and apply the following rules (in order) to expand until it fits.
-            <ol style="list-style-type:lower-alpha">
-              <li>Expand space characters up to the maximum value specified by 'word-spacing' property, or up to 1/2em.</li>
-              <li>Expand spaces created by 'text-spacing' property up to 1/2em.</li>
-              <li>Expand all expansion opportunities as defined above in equal percent of the size of each character.</li>
-            </ol></li>
-        </ol>
-      --></div>
+  <p>The UA may enable or break optional ligatures or use other font features
+   such as alternate glyphs or glyph compression to help justify the text
+   under any method. This behavior is not controlled by this level of CSS.
+   However, UAs <em>must not</em> break required ligatures or otherwise
+   disable features required to correctly shape complex scripts.
 
   <h2 id=spacing><span class=secno>8. </span> Spacing</h2>