tighten font feature values rule parsing and add issue for default features

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-6-july-2012>Editor's Draft 6
+   <h2 class="no-num no-toc" id=editors-draft-28-july-2012>Editor's Draft 28
     July 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-20120706/">http://www.w3.org/TR/2012/ED-css3-fonts-20120706/</a> -->
+     <!-- <dd><a href="http://www.w3.org/TR/2012/ED-css3-fonts-20120728/">http://www.w3.org/TR/2012/ED-css3-fonts-20120728/</a> -->
 
 
     <dt>Latest version:
@@ -4027,13 +4027,18 @@ <h3 id=font-feature-values><span class=secno>6.9 </span>Defining font
    class=prod><dfn id=ltfeature-indexgt><var>&lt;feature-index&gt;</var></dfn>                   = an integer value greater than 0 specific to a given font or set of fonts</pre>
 
   <p>These @font-feature-values rules can appear anywhere within a stylesheet
-   and are exposed bidirectionally across @import boundaries. If multiple
-   @font-feature-values rules are defined for a given family, the resulting
-   values defined are the union of these rules. This allows a set of
-   named-values to be defined for a given font family globally for a site and
-   specific additions made per-page. If the same &lt;feature-value-name&gt;
-   is defined mulitple times for a given font-variant value, the last defined
-   value is used.
+   and are exposed bidirectionally across @import boundaries. Within each
+   font feature values declaration, syntax errors such as unknown property
+   value names, invalid identifiers or values result in the declaration being
+   dropped, similar to the way syntax errors in style declarations are
+   handled.
+
+  <p>If multiple @font-feature-values rules are defined for a given family,
+   the resulting values defined are the union of these rules. This allows a
+   set of named-values to be defined for a given font family globally for a
+   site and specific additions made per-page. If the same
+   &lt;feature-value-name&gt; is defined mulitple times for a given
+   font-variant value, the last defined value is used.
 
   <pre>
 site.css:
@@ -4057,9 +4062,10 @@ <h3 id=font-feature-values><span class=secno>6.9 </span>Defining font
 
   <p>Only named font families are allowed for &lt;font-family&gt;, rules that
    include generic or system fonts in the list of font families are
-   considered syntax errors and ignored. However, if a user agent defines a
-   generic font to be a specific named font (e.g. Helvetica), the settings
-   associated with that family name will be used.
+   considered syntax errors and the contents of the rules are ignored.
+   However, if a user agent defines a generic font to be a specific named
+   font (e.g. Helvetica), the settings associated with that family name will
+   be used.
 
   <p>For &lt;font-variant-property-value>, only font specific property value
    names supported by the ‘<a href="#propdef-font-variant"><code
@@ -4099,7 +4105,8 @@ <h3 id=font-feature-values><span class=secno>6.9 </span>Defining font
   <p>For the styleset property value, multiple values indicate the style sets
    to be enabled. Values between 1 and 20 enable OpenType features ‘<code
    class=css>ss01</code>’ through ‘<code class=css>ss20</code>’. For
-   OpenType fonts, values greater than 20 are ignored.
+   OpenType fonts, values greater than 20 or 0 are ignored but do not
+   generate a syntax error when parsed.
 
   <pre>@font-feature-values Mars Serif {
   @styleset alt-g 1,        /* implies ss01 = 1 */
@@ -4117,10 +4124,11 @@ <h3 id=font-feature-values><span class=secno>6.9 </span>Defining font
   <p>For character-variant, a single value between 1 and 99 indicates the
    enabling of OpenType feature ‘<code class=css>cv01</code>’ through
    ‘<code class=css>cv99</code>’. For OpenType fonts, values greater than
-   99 are ignored. When two values are listed, the first value indicates the
-   feature used and the second the value passed for that feature. When two
-   value names imply different settings for the same underlying feature the
-   last setting is used.
+   99 or 0 are ignored but do not generate a syntax error when parsed. When
+   two values are listed, the first value indicates the feature used and the
+   second the value passed for that feature. When two value names imply
+   different settings for the same underlying feature the last setting is
+   used.
 
   <pre>@font-feature-values MM Greek {
   @character-variant alpha-2 1 2;   /* implies cv01 = 2 */
@@ -4659,6 +4667,13 @@ <h2 id=font-feature-resolution><span class=secno>7 </span>Font feature
    href="http://www.microsoft.com/typography/otfntdev/devanot/features.aspx">Devanagari</a>
    additional features are required.
 
+  <p class=issue>Concern has been expressed that requiring default features
+   to be enabled by default will result in text rendering performance
+   degradation. One proposed alternative is to add a new default ‘<code
+   class=property>auto</code>’ value for font-variant and allow user agents
+   to determine whether to enable default features or not when that value is
+   used.
+
   <p>General and font specific font feature property settings are resolved in
    the order below, in ascending precedence order. This ordering is used to
    construct a combined list of font features that affect a given text run.