update font-variant-position definition

Author: John Daggett

css3-fonts/Fonts.html

@@ -120,15 +120,15 @@
 
    <h1>CSS Fonts Module Level 3</h1>
 
-   <h2 class="no-num no-toc" id=editors-draft-18-may-2012>Editor's Draft 18
+   <h2 class="no-num no-toc" id=editors-draft-24-may-2012>Editor's Draft 24
     May 2012</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/2012/ED-css3-fonts-20120518/">http://www.w3.org/TR/2012/ED-css3-fonts-20120518/</a> -->
+     <!-- <dd><a href="http://www.w3.org/TR/2012/ED-css3-fonts-20120524/">http://www.w3.org/TR/2012/ED-css3-fonts-20120524/</a> -->
 
 
     <dt>Latest version:
@@ -383,8 +383,8 @@ <h2 class="no-num no-toc" id=contents>Table of contents</h2>
 
    <li class=no-num><a href="#ch-ch-ch-changes">Changes</a>
     <ul class=toc>
-     <li class=no-num><a href="#recent-changes"> Changes from the March 2011
-      CSS3 Fonts Working Draft</a>
+     <li class=no-num><a href="#recent-changes"> Changes from the October
+      2011 CSS3 Fonts Working Draft</a>
     </ul>
 
    <li class=no-num><a href="#acknowledgments">Acknowledgments</a>
@@ -3257,14 +3257,12 @@ <h3 id=font-variant-position-prop><span class=secno>6.4 </span>Subscript
      <td>as specified
   </table>
 
-  <p class=issue>The name for this feature has been a point of contention,
-   this name is a placeholder until the final name is resolved.
-
-  <p>This property controls the selection of alternate glyphs for use as
-   subscripts and superscripts. These alternate glyphs of reduced size are
-   positioned within the same em-box as the default glyph and explicitly
-   designed to match the surrounding text and make the them more readable
-   without affecting the linebox.
+  <p>This property is used to enable typographic subscript and superscript
+   glyphs. These are alternate glyphs designed within the same em-box as
+   default glyphs and are intended to be laid out on the same baseline as the
+   default glyphs, with no resizing or repositioning of the baseline. They
+   are explicitly designed to match the surrounding text and to be more
+   readable without affecting the line height.
 
   <div class=figure><img alt="comparison between real subscript glyphs and
    synthesized ones" src=realsubscripts.png>
@@ -3275,13 +3273,18 @@ <h3 id=font-variant-position-prop><span class=secno>6.4 </span>Subscript
   <p>The values ‘<code class=property>sub</code>’ and ‘<code
    class=property>super</code>’ imply the appropriate variant glyph is
    displayed when available in the font (OpenType features: <code>subs,
-   supr</code>). Because of the semantic nature of subscripts and
-   superscripts, when the value is either ‘<code
-   class=property>sub</code>’ or ‘<code class=property>super</code>’
-   and a variant glyph is not available, a simulated glyph is synthesized
-   using a reduced form of the default glyph. A value of ‘<a
-   href="#normal2"><code class=property>normal</code></a>’ implies use of
-   the default glyph at normal size.
+   supr</code>). A value of ‘<a href="#normal2"><code
+   class=property>normal</code></a>’ implies neither of these alternate
+   glyphs are substituted.
+
+  <p>Because of the semantic nature of subscripts and superscripts, when the
+   value is either ‘<code class=property>sub</code>’ or ‘<code
+   class=property>super</code>’ for a given run of text and a variant glyph
+   is not available for all the characters in the run, simulated glyphs are
+   synthesized for all characters using reduced forms of the default glyph.
+   This is done to avoid a mixture of variant glyphs and synthesized ones
+   within the same run of text, since there is no guarantee that two types of
+   glyphs would be aligned correctly.
 
   <p>In the case of OpenType fonts that lack subscript or superscript glyphs
    for a given character, to calculate the size and offset of the synthesized
@@ -3297,26 +3300,39 @@ <h3 id=font-variant-position-prop><span class=secno>6.4 </span>Subscript
    subscripts and superscripts, it is recommended that authors use
    conditional rules <a href="#CSS3-CONDITIONAL"
    rel=biblioentry>[CSS3-CONDITIONAL]<!--{{CSS3-CONDITIONAL}}--></a> so that
-   older user agents will still render these correctly.
-
-  <p>Note that because this property is not cumulative, it's not suitable for
-   nested subscripts or superscripts. Due to this limitation,
-   font-variant-position is not recommended for use in user agent
-   stylesheets.
-
-  <p class=issue>It would be nice to define this in a way such that it would
-   work with nested subscripts or superscripts which would allow it to be
-   used in user agent stylesheets. But one of the goals for this is to allow
-   simple subscripts and superscripts to be displayed without affecting the
-   line box. To achieve both appears to be a very hairy problem. One <a
-   href="http://lists.w3.org/Archives/Public/www-style/2011Apr/0391.html">proposal</a>
-   is to conditionalize how the font-size and vertical-align properties are
-   used based on whether subscript or superscript glyphs are available.
-
-  <p class=issue>Fonts often include default values for subscript/superscript
-   metrics. As a result, a mixture of designed glyphs and simulated glyphs
-   may not display correctly, the baseline will appear to bounce between
-   sequences to designed and simulated glyphs.
+   older user agents will still render subscripts and superscripts via the
+   older mechanism.
+
+  <p>Authors should note that fonts typically only provide subscript and
+   superscript glyphs for a subset of all characters supported by the font.
+   While subscripts and superscript glyphs are often available for Latin
+   numbers, punctuation and letter characters are less frequently supported.
+   The synthetic fallback rules defined for this property assure that
+   subscripts and superscripts will always appear but the appearance may not
+   match author expectations if the font used doesn't provide the appropriate
+   alternate glyph for all characters contained in a subscript or
+   superscript.
+
+  <div class=figure><img alt="alternate superscripts vs. glyphs synthesized
+   using superscript metrics" src=superscript-alt-synth.png>
+   <p class=caption>Superscript alternate glyph (left), synthesized
+    superscript glyphs (middle), and incorrect mixture of the two (right)
+  </div>
+
+  <p>This property is not cumulative, applying it to subelements within a
+   subscript or superscript won't nest the placement of a subscript or
+   superscript glyph. Images contained within text runs where the value of
+   this property is ‘<code class=property>sub</code>’ or ‘<code
+   class=property>super</code>’ will be drawn just as they would if the
+   value was ‘<a href="#normal2"><code class=property>normal</code></a>’.
+   Likewise, text decorations such as underlines or emphasis marks will
+   render in the same position as they would for the default glyphs, since
+   this property does not affect the baseline position.
+
+  <p>Because of these limitations, font-variant-position is not recommended
+   for use in user agent stylesheets. Authors should use it in cases where
+   subscripts or superscripts will only contain the narrow range of
+   characters supported by the fonts specified.
 
   <div class=example>
    <p>A typical user agent default style for the <code>sub</code> element:</p>
@@ -4903,21 +4919,28 @@ <h2 class=no-num id=font-feature-values-DOM>Appendix B: DOM Interfaces</h2>
   <h2 class=no-num id=ch-ch-ch-changes>Changes</h2>
 
   <h3 class=no-num id=recent-changes> Changes from the <a
-   href="http://www.w3.org/TR/2011/WD-css3-fonts-20110324/">March 2011 CSS3
+   href="http://www.w3.org/TR/2011/WD-css3-fonts-20111004/">October 2011 CSS3
    Fonts Working Draft</a></h3>
 
   <p>Major changes include:
 
   <ul>
-   <li>Made same-origin wording normative and marked it at-risk
+   <li>Based on TPAC resolution, same-origin restriction is normative
+
+   <li>Added parameter specifics for using CORS with font loads
+
+   <li>Revised fallback handling of subscript/superscript variants
 
-   <li>Simplified definition of subscript/superscript variant property
+   <li>Switched to using CSS3 Values and Units hash notation for
+    comma-delimited lists
 
-   <li>Trimmed some font-variant property values and renamed others
+   <li>Reworked font-family syntax to be clearer about the use of keywords in
+    unquoted font family names
 
-   <li>Added more examples for various font variant value types
+   <li>Tightened the syntax of font-feature-settings to only allow
+    four-character tags
 
-   <li>Added DOM interface definitions for @font-feature-values rules
+   <li>Added missing definitions of default values for some properties
   </ul>
 
   <h2 class=no-num id=acknowledgments>Acknowledgments</h2>
@@ -4929,8 +4952,10 @@ <h2 class=no-num id=acknowledgments>Acknowledgments</h2>
    Lunde and Eric Muller provided valuable feedback on CJK OpenType features
    and Unicode variation selectors. The idea for supporting font features by
    using font-variant subproperties originated with Håkon Wium Lie, Adam
-   Twardoch and Tal Leming. Thanks also to House Industries for allowing the
-   use of Ed Interlock in the discretionary ligatures example.
+   Twardoch and Tal Leming. Elika Etemad supplied some of the initial design
+   ideas for the @font-feature-values rule. Thanks also to House Industries
+   for allowing the use of Ed Interlock in the discretionary ligatures
+   example.
 
   <p>A special thanks to Robert Bringhurst for the sublime mind expansion
    that is <em>The Elements of Typographic Style</em>.
@@ -5046,11 +5071,11 @@ <h3 class=no-num id=normative-references>Normative References</h3>
    <dt id=UNICODE6>[UNICODE6]
 
    <dd>The Unicode Consortium. <a
-    href="http://www.unicode.org/versions/Unicode6.0.0/"><cite>The Unicode
-    Standard, Version 6.0.0.</cite></a> Defined by: The Unicode Standard,
-    Version 6.0.0 (Mountain View, CA: The Unicode Consortium, 2011. ISBN
-    978-1-936213-01-6) URL: <a
-    href="http://www.unicode.org/versions/Unicode6.0.0/">http://www.unicode.org/versions/Unicode6.0.0/</a>
+    href="http://www.unicode.org/versions/Unicode6.1.0/"><cite>The Unicode
+    Standard, Version 6.1.0.</cite></a> Defined by: The Unicode Standard,
+    Version 6.1.0 (Mountain View, CA: The Unicode Consortium, 2012. ISBN
+    978-1-936213-02-3) URL: <a
+    href="http://www.unicode.org/versions/Unicode6.1.0/">http://www.unicode.org/versions/Unicode6.1.0/</a>
     </dd>
    <!---->
   </dl>