[css-text-3] Disentangle content language and writing system

Closes #2015

Author: frivoal

css-fonts-4/Overview.bs

@@ -3448,8 +3448,10 @@ traditions found in Spanish, Italian and French orthography:
 If the content language of the element is known according to the
 rules of the <a href="https://www.w3.org/TR/CSS21/conform.html#doclanguage">document language</a>,
 user agents are required to infer the OpenType language system from
-the content language and use that when selecting and positioning
+the [=content language=] and use that when selecting and positioning
 glyphs using an OpenType font.
+If a [=writing system=] has been explicitely specified,
+it must take precedence over the customary one implied by the [=content language=].
 
 <!-- previously in level 3, now moved to Level 4 -->
 For OpenType fonts, in some cases it may be necessary to explicitly

css-text-3/Overview.bs

@@ -180,6 +180,16 @@ Languages and Typesetting</h3>
     the universal <code>xml:lang</code> attribute in XML,
     and the HTTP <code>Content-Language</code> header for content served over HTTP.
 
+    The [=content language=] an element is declared to be in
+    also identifies the specific written form of that language used in that element,
+    known as the <dfn export>writing system</dfn>.
+
+    Note: Depending on the [=document language=]'s facilities for identifying the [=content language=],
+    information about the [=writing system=] may only be carried implicitly.
+    That is typically the case with the [[BCP47]] language tag used in [[HTML]],
+    although it can optionally indicate the [=writing system=] explicitly
+    using a script subtag.
+
     Language and writing system conventions can affect
     line breaking, hyphenation, justification, glyph selection,
     and many other typographic effects.
@@ -391,6 +401,7 @@ Characters and Letters</h3>
         replaced with a different set of mappings to their respective
         undotted/dotted counterparts, which do not exist in English. This
         mapping must only take effect if the <a>content language</a> is Turkish
+        written in its customary Latin-based <a>writing system</a>
         (or another Turkic language that uses Turkish casing rules);
         in other languages, the usual mapping of &ldquo;I&rdquo;
         and &ldquo;i&rdquo; is required. This rule is thus conditionally
@@ -726,8 +737,8 @@ Characters and Letters</h3>
           <code>W</code>, or <code>H</code> (not <code>A</code>),
           and neither side is Hangul or Emoji (Unicode property <code>Emoji</code>),
           then the segment break is removed.
-        <li>Otherwise, if the <a>content language</a> of the <a>segment break</a>
-          is Chinese, Japanese, or Yi,
+        <li>Otherwise, if the <a>writing system</a> of the <a>segment break</a>
+          is <a for=writing-system>Chinese</a>, <a for=writing-system>Japanese</a>, or Yi,
           and the character before or after the segment break
           is punctuation or a symbol (Unicode <a>general category</a> P* or S*)
           and has an <a>East Asian Width property</a> of <code>A</code>
@@ -1166,7 +1177,7 @@ Line Breaking Details</h3>
         </ul>
       <li>
         The following breaks are allowed for ''line-break/normal'' and ''loose'' line breaking
-        if the <a>content language</a> is Chinese or Japanese,
+        if the <a>writing system</a> is <a for=writing-system>Chinese</a> or <a for=writing-system>Japanese</a>,
         and are otherwise forbidden:
         <ul>
           <li>breaks before hyphens:<br>
@@ -1187,7 +1198,7 @@ Line Breaking Details</h3>
         </ul>
       <li>
         The following breaks are allowed for ''loose''
-        if the <a>content language</a> is Chinese or Japanese
+        if the <a>writing system</a> is <a for=writing-system>Chinese</a> or <a for=writing-system>Japanese</a>
         and are otherwise forbidden:
         <ul>
           <li>breaks before certain centered punctuation marks:<br>
@@ -1208,7 +1219,7 @@ Line Breaking Details</h3>
     <p class="note">In the requirements listed above,
       no distinction is made among the levels of strictness in non-CJK text:
       only CJK codepoints are affected,
-      unless the text is marked as Chinese or Japanese,
+      unless the text is marked as <a for=writing-system>Chinese</a> or <a for=writing-system>Japanese</a>,
       in which case some additional common codepoints are affected.
 
     <div class="example">
@@ -1562,7 +1573,7 @@ Shaping Across Intra-word Breaks</h3>
       <dt><dfn>auto</dfn></dt>
         <dd>The UA determines the justification algorithm to follow, based
           on a balance between performance and adequate presentation quality.
-          Since justification rules vary by writing system and language,
+          Since justification rules vary by [=writing system=],
           UAs should, where possible, use a justification algorithm appropriate to the text.
 
           <p class="example">
@@ -1571,11 +1582,11 @@ Shaping Across Intra-word Breaks</h3>
           primarily expanding <a>word separators</a>
           and between CJK <a>typographic letter units</a>
           along with secondarily expanding between Southeast Asian <a>typographic letter units</a>.
-          Then, in cases where the <a>content language</a> of the paragraph is known,
+          Then, in cases where the <a>writing system</a> of the paragraph is <a for=writing-system>known</a>,
           it could choose a more language-tailored justification behavior
           e.g. following [[JLREQ]] for Japanese,
           using cursive elongation for Arabic,
-          using ''inter-word'' for German,
+          using ''inter-word'' for Latin,
           etc.
 
         <div class="figure" id="fig-text-justify-cursive">
@@ -2506,7 +2517,7 @@ Appendix D: Scripts and Spacing</h2>
   	  The following <a>Unicode scripts</a> are included:
       Bopomofo, Han, Hangul, Hiragana, Katakana, and Yi.
   	  Characters of the <a>East Asian Width property</a> <code>W</code> and <code>F</code> are also included,
-  	  but <code>A</code> characters are included only if the <a>content language</a> is Chinese, Korean, or Japanese.
+  	  but <code>A</code> characters are included only if the <a>writing system</a> is <a for=writing-system>Chinese</a>, <a for=writing-system>Korean</a>, or <a for=writing-system>Japanese</a>.
   	<dt><dfn>clustered scripts</dfn></dt>
   	<dd>Clustered scripts have discrete units
       and break only at word boundaries,
@@ -2579,6 +2590,8 @@ Characters and Properties</h2>
 <h2 id="script-tagging" class="no-num">Appendix F.
 Tagging Content by Writing System</h2>
 
+	<p><em>This appendix is normative.</em></p>
+
 	While most languages have a preferred writing system,
 	many can also be transcribed into a different writing system.
 	As a common example, most languages have at least one Latin transcription,
@@ -2591,7 +2604,8 @@ Tagging Content by Writing System</h2>
 	does not use word spaces,
 	and should therefore be typeset as for Chinese.
 
-	Authors can indicate the use of an atypical writing system
+	In [[HTML]] or any other <a>document language</a> using [[BCP47]] to identify the [=content language=],
+	authors can indicate the use of an atypical writing system
 	with script subtags.
 	For example, to indicate use of the Latin writing system
 	for languages which don't natively use it,
@@ -2629,6 +2643,26 @@ Tagging Content by Writing System</h2>
 	not the conventions of that language in a different writing system,
 	which would be inappropriate to the writing system used in this case.
 
+	The full correspondence between languages and their most common writing system
+	is out of scope for this document.
+	However, User Agents must assume at least the following:
+
+	* If the [=content language=] is Chinese and the [=writing system=] is unspecified,
+		or for any [=content language=] if the [=writing system=] to specified to be one of the ''Hant'', ''Hans'', ''Hani'', ''Hanb'', or ''Bopo'' [[ISO15924]] codes,
+		then the [=writing system=] is <dfn no-export for=writing-system>Chinese</dfn>.
+	* If the [=content language=] is Japanese and the [=writing system=] is unspecified,
+		or for any [=content language=] if the [=writing system=] to specified to be one of the ''Jpan'', ''Hrkt'', ''Hira'' or ''Kana'' [[ISO15924]] codes,
+		then the [=writing system=] is <dfn no-export for=writing-system>Japanese</dfn>.
+	* If the [=content language=] is Korean and the [=writing system=] is unspecified,
+		or for any [=content language=] if the [=writing system=] to specified to be one of the ''Kore'', ''Hang'', or ''Jamo'' [[ISO15924]] codes,
+		then the [=writing system=] is <dfn no-export for=writing-system>Korean</dfn>.
+	* The [=writing system=] is only considered to be <dfn for=writing-system lt='known | unknown'>unknown</dfn>
+		if the [=content language=] itself is unknown,
+		or if it explicitly indicates an unknown writing system.
+
+		Note: Mere omission of the [=writing system=] information when the [=content language=] is specified
+		means the that the [=writing system=] is implied, not unknown.
+
 	More advice on language tagging can be found in
 	the <a href="https://www.w3.org/International/core/">Internationalization Working Group</a>’s
 	<a href="https://www.w3.org/International/articles/language-tags/">“Language tags in HTML and XML”</a>