revised vertical-position

Author: John Daggett

css3-fonts/Fonts.html

@@ -111,14 +111,14 @@
 
    <h1>CSS Fonts Module Level 3</h1>
 
-   <h2 class="no-num no-toc" id=editors>Editor's Draft 24 February 2011</h2>
+   <h2 class="no-num no-toc" id=editors>Editor's Draft 1 March 2011</h2>
 
    <dl id=authors>
     <dt>This version:
 
     <dd><a
      href="http://dev.w3.org/csswg/css3-fonts/">http://dev.w3.org/csswg/css3-fonts/</a>
-     <!-- <dd><a href="http://www.w3.org/TR/2011/ED-css3-fonts-20110224">http://www.w3.org/TR/2011/ED-css3-fonts-20110224</a> -->
+     <!-- <dd><a href="http://www.w3.org/TR/2011/ED-css3-fonts-20110301">http://www.w3.org/TR/2011/ED-css3-fonts-20110301</a> -->
 
 
     <dt>Latest version:
@@ -2770,16 +2770,24 @@ <h3 id=font-style-matching><span class=secno>5.1 </span>Matching font
   <h3 id=char-handling-issues><span class=secno>5.2 </span>Character handling
    issues</h3>
 
-  <p>The procedure above does not assume that text runs are in either
-   normalized or denormalized form <a href="#UAX15"
-   rel=biblioentry>[UAX15]<!--{{!UAX15}}--></a>. Layout engines often convert
-   base character plus combining character sequences into precomposed
-   codepoints if they exist. Fonts can generally support both ways of
-   matching characters but variations can occur. Authors should always tailor
-   their choice of fonts to their content, including whether that content
-   contains normalized or denormalized character streams.
-
-  <p>If a text run contains variation selectors, special handling is
+  <p>The procedure above is always performed on text runs containing Unicode
+   characters, documents using legacy encodings are assumed to have been
+   transcoded before matching fonts. For fonts containing character maps for
+   both legacy encodings and Unicode, the contents of the legacy encoding
+   character map must have no effect on the results of the font matching
+   process.
+
+  <p>The font matching process does not assume that text runs are in either
+   normalized or denormalized form (see <a href="#CHARMOD-NORM"
+   rel=biblioentry>[CHARMOD-NORM]<!--{{!CHARMOD-NORM}}--></a> for more
+   details). Layout engines often convert base character plus combining
+   character sequences into precomposed characters if they exist. Fonts can
+   generally support both ways of matching characters but variations can
+   occur. Authors should always tailor their choice of fonts to their
+   content, including whether that content contains normalized or
+   denormalized character streams.
+
+  <p>If a text run contains Unicode variation selectors, special handling is
    required. For each character + variation selector pair, if the first font
    with a glyph for the base character also contains a glyph for the variant
    specified by the variation selector, user agents must display the variant
@@ -3104,18 +3112,73 @@ <h3 id=vertical-position-prop><span class=secno>6.4 </span>Subscript,
 
   <p class=issue>Issue: the name for this feature has been a point of
    contention. The name used here is taken from the AAT feature name but the
-   eventual name for this is still under discussion.
+   eventual name may change.
 
   <p>The values ‘<code class=property>subscript</code>’, ‘<code
-   class=property>superscript</code>’ and ‘<code
+   class=property>superscript</code>’, and ‘<code
    class=property>ordinal</code>’ imply the appropriate variant glyph is
    displayed when available in the font (OpenType features: <code>subs, supr,
-   ordn</code>). When a variant glyph is not available, a simulated version
-   is synthesized using a reduced form of the default glyph. Normal implies
-   use of the default glyph at normal size. When the value is anything other
-   than ‘<code class=property>normal</code>’, the font-size and
-   vertical-align properties are set to ‘<code
-   class=property>inherit</code>’.</p>
+   ordn</code>). Because of the semantic nature of subscripts and
+   superscripts, when the value is either ‘<code
+   class=property>subscript</code>’ or ‘<code
+   class=property>superscript</code>’ and a variant glyph is not available,
+   a simulated glyph is synthesized using a reduced form of the default
+   glyph. If the value is ‘<code class=property>ordinal</code>’ and a
+   variant glyph is not available, the normal glyph is used, no attempt is
+   made to synthesize one. A value of ‘<code
+   class=property>normal</code>’ implies use of the default glyph at normal
+   size.
+
+  <p>In the past, user agents have used font-size and vertical-align to
+   simulate subscripts and superscripts for the <code>sub</code> and
+   <code>sup</code> elements. To allow a backwards combatible way of defining
+   subscripts and superscripts, when the value is either ‘<code
+   class=property>subscript</code>’ or ‘<code
+   class=property>superscript</code>’, the property acts as a shorthand and
+   resets the value of the font-size and vertical-align properties to
+   ‘<code class=property>inherit</code>’. Other than this, the property
+   does not affect the height of the line box.
+
+  <div class=example>
+   <p>A typical user agent default style for the <code>sub</code> element:</p>
+
+   <pre>sub {
+  vertical-align: sub;
+  font-size: smaller;
+  line-height: normal;
+}
+</pre>
+
+   <p>Using vertical-position to specify typographic subscripts in a way that
+    will still show subscripts in older user agents:</p>
+
+   <pre>sub {
+  vertical-align: sub;
+  font-size: smaller;
+  line-height: normal;
+  vertical-position: subscript;
+}
+</pre>
+
+   <p>User agents that support the ‘<code class=property><a
+    href="#propdef-vertical-position">vertical-position</a></code>’
+    property will select a subscript variant glyph and reset vertical-align
+    and font-size to ‘<code class=property>inherit</code>’. Older user
+    agents will ignore the vertical-position property definition and use the
+    standard defaults for subscripts.</p>
+  </div>
+
+  <p>Note that because this property is not cumulative, it's not suitable for
+   nested subscripts or superscripts. Due to this limitation,
+   vertical-position is not recommended for use in user agent stylesheets.
+
+  <p>In the case of OpenType fonts that lack subscript or superscript glyphs,
+   to calculate the size and offset of the synthesized substitutes user
+   agents must use the appropriate subscript and superscript metrics
+   specified in the selected font's <a
+   href="http://www.microsoft.com/typography/otspec/os2.htm#subxs">OS/2
+   table</a> <a href="#OPENTYPE"
+   rel=biblioentry>[OPENTYPE]<!--{{OPENTYPE}}--></a>.</p>
   <!-- prop: font-variant-ligatures -->
 
   <h3 id=font-variant-ligatures-prop><span class=secno>6.5 </span>Ligatures:
@@ -4547,13 +4610,33 @@ <h3 class=no-num id=normative-references>Normative References</h3>
     </dd>
    <!---->
 
+   <dt id=CHARMOD>[CHARMOD]
+
+   <dd>Martin J. Dürst; et al. <a
+    href="http://www.w3.org/TR/2005/REC-charmod-20050215"><cite>Character
+    Model for the World Wide Web 1.0: Fundamentals.</cite></a> 15 February
+    2005. W3C Recommendation. URL: <a
+    href="http://www.w3.org/TR/2005/REC-charmod-20050215">http://www.w3.org/TR/2005/REC-charmod-20050215</a>
+    </dd>
+   <!---->
+
+   <dt id=CHARMOD-NORM>[CHARMOD-NORM]
+
+   <dd>François Yergeau; et al. <a
+    href="http://www.w3.org/TR/2005/WD-charmod-norm-20051027"><cite>Character
+    Model for the World Wide Web 1.0: Normalization.</cite></a> 27 October
+    2005. W3C Working Draft. (Work in progress.) URL: <a
+    href="http://www.w3.org/TR/2005/WD-charmod-norm-20051027">http://www.w3.org/TR/2005/WD-charmod-norm-20051027</a>
+    </dd>
+   <!---->
+
    <dt id=CORS>[CORS]
 
    <dd>Anne van Kesteren. <a
-    href="http://www.w3.org/TR/2010/WD-cors-20100727"><cite>Cross-Origin
+    href="http://www.w3.org/TR/2010/WD-cors-20100727/"><cite>Cross-Origin
     Resource Sharing.</cite></a> 27 July 2010. W3C Working Draft. (Work in
     progress.) URL: <a
-    href="http://www.w3.org/TR/2010/WD-cors-20100727">http://www.w3.org/TR/2010/WD-cors-20100727</a>
+    href="http://www.w3.org/TR/2010/WD-cors-20100727/">http://www.w3.org/TR/2010/WD-cors-20100727/</a>
     </dd>
    <!---->
 
@@ -4598,19 +4681,20 @@ <h3 class=no-num id=normative-references>Normative References</h3>
    <dt id=UAX15>[UAX15]
 
    <dd>Mark Davis. <a
-    href="http://www.unicode.org/reports/tr15/"><cite>Unicode Normalization
-    Forms.</cite></a> 17 September 2010. Unicode Standard Annex #15. URL: <a
-    href="http://www.unicode.org/reports/tr15/">http://www.unicode.org/reports/tr15/</a>
+    href="http://www.unicode.org/reports/tr15/tr15-33.html"><cite>Unicode
+    Normalization Forms.</cite></a> 17 September 2010. Unicode Standard Annex
+    #15. URL: <a
+    href="http://www.unicode.org/reports/tr15/tr15-33.html">http://www.unicode.org/reports/tr15/tr15-33.html</a>
     </dd>
    <!---->
 
    <dt id=UAX29>[UAX29]
 
    <dd>Mark Davis. <a
-    href="http://www.unicode.org/reports/tr29/"><cite>Unicode Text
-    Segmentation.</cite></a> 8 October 2010. Unicode Standard Annex #29. URL:
-    <a
-    href="http://www.unicode.org/reports/tr29/">http://www.unicode.org/reports/tr29/</a>
+    href="http://www.unicode.org/reports/tr29/tr29-17.html"><cite>Unicode
+    Text Segmentation.</cite></a> 8 October 2010. Unicode Standard Annex #29.
+    URL: <a
+    href="http://www.unicode.org/reports/tr29/tr29-17.html">http://www.unicode.org/reports/tr29/tr29-17.html</a>
     </dd>
    <!---->
 
@@ -4634,6 +4718,8 @@ <h3 class=no-num id=normative-references>Normative References</h3>
   <!--{{!UAX29}}-->
   <!--{{!CORS}}-->
   <!--{{!HTML5}}-->
+  <!--{{!CHARMOD}}-->
+  <!--{{!CHARMOD-NORM}}-->
 
   <h3 class=no-num id=other-references>Other References</h3>
   <!--begin-informative-->
@@ -4674,6 +4760,15 @@ <h3 class=no-num id=other-references>Other References</h3>
     ISBN 1-932026-01-0.</dd>
    <!---->
 
+   <dt id=OPENTYPE>[OPENTYPE]
+
+   <dd><a
+    href="http://www.microsoft.com/typography/otspec/default.htm"><cite>OpenType
+    specification.</cite></a> Microsoft. URL: <a
+    href="http://www.microsoft.com/typography/otspec/default.htm">http://www.microsoft.com/typography/otspec/default.htm</a>
+    </dd>
+   <!---->
+
    <dt id=OPENTYPE-FONT-GUIDE>[OPENTYPE-FONT-GUIDE]
 
    <dd><a