[css-fonts] remove requirement to use often inaccurate OS/2 table subscript/superscript metrics

Author: John Daggett

css-fonts/Fonts.html

@@ -2459,8 +2459,10 @@ <h3 id=src-desc><span class=secno>4.3 </span>Font reference: the <a
    unique font face name enclosed by "local(" and ")". The name can
    optionally be enclosed in quotes. If unquoted, the unquoted font family
    name processing conventions apply; the name must be a sequence of
-   identifiers separated by whitespace which is converted to a string by
-   joining the identifiers together separated by a single space.
+   identifiers separated by <a
+   href="http://www.w3.org/TR/css3-syntax/#whitespace">whitespace</a> which
+   is converted to a string by joining the identifiers together separated by
+   a single space.
 
   <pre>
 /* regular face of Gentium */
@@ -4036,12 +4038,8 @@ <h3 id=font-variant-position-prop><span class=secno>6.5 </span>Subscript
    this feature applied. This is done per run to avoid a mixture of variant
    glyphs and synthesized ones that would not align correctly. In the case of
    OpenType fonts that lack subscript or superscript glyphs for a given
-   character, user agents must use the appropriate subscript and superscript
-   metrics specified in the selected font's <a
-   href="https://www.microsoft.com/typography/otspec/os2.htm#subxs">OS/2
-   table</a> <a href="#OPENTYPE"
-   rel=biblioentry>[OPENTYPE]<!--{{!OPENTYPE}}--></a> to calculate the size
-   and offset of the synthesized substitutes.
+   character, user agents must synthesize appropriate subscript and
+   superscript glyphs.
 
   <div class=figure><img alt="alternate superscripts vs. glyphs synthesized
    using superscript metrics" src=superscript-alt-synth.png>
@@ -4063,6 +4061,23 @@ <h3 id=font-variant-position-prop><span class=secno>6.5 </span>Subscript
    older user agents will still render subscripts and superscripts via the
    older mechanism.
 
+  <p>Because <code>font-size: smaller</code> is often used for these
+   elements, the effective scaling factor applied to subscript and
+   superscript text varies depending upon the size. For larger text, the font
+   size is often reduced by a third but for smaller text sizes, the reduction
+   can be much less. This allows subscripts and superscripts to remain
+   readable even within elements using small text sizes. User agents should
+   consider this when deciding how to synthesize subscript and superscript
+   glyphs.
+
+  <p class=note>The OpenType font format defines subscript and superscript
+   metrics in the <a
+   href="https://www.microsoft.com/typography/otspec/os2.htm#subxs">OS/2
+   table</a> <a href="#OPENTYPE"
+   rel=biblioentry>[OPENTYPE]<!--{{!OPENTYPE}}--></a> but these are not
+   always accurate in practice and so cannot be relied upon when synthesizing
+   subscript and superscript glyphs.
+
   <p>Authors should note that fonts typically only provide subscript and
    superscript glyphs for a subset of all characters supported by the font.
    For example, while subscript and superscript glyphs are often available
@@ -5576,7 +5591,7 @@ <h3 id=font-language-override-prop><span class=secno>6.13 </span>Font
     system tag</a>, specifies the OpenType language system to be used instead
     of the language system implied by the language of the element
   </dl>
-  <!-- For this level, string represents a single language code, there's no concept of "fallback" 
+  <!-- For this level, string represents a single language code, there's no concept of "fallback"
 https://www.w3.org/2013/06/07-css-minutes.html#item04 -->
 
   <p>Use of invalid OpenType language system tags must not generate a parse
@@ -6046,6 +6061,9 @@ <h3 class=no-num id=recent-changes> Changes from the <a
    <li>make negative values for font-size-adjust invalid, along with negative
     percentage font-size values
 
+   <li>remove the requirement that user agents use OS/2 table
+    subscript/superscript metrics
+
    <li>minor editorial cleanups
   </ul>
 

css-fonts/Fonts.src.html

@@ -1698,8 +1698,9 @@ <h3 id="src-desc">Font reference: the <a href="#descdef-src">src</a> descriptor<
 name enclosed by "local(" and ")".  The name can optionally be
 enclosed in quotes.  If unquoted, the unquoted font family name
 processing conventions apply; the name must be a sequence of
-identifiers separated by whitespace which is converted to a string by
-joining the identifiers together separated by a single space.</p>
+identifiers separated by <a href="http://www.w3.org/TR/css3-syntax/#whitespace">whitespace</a>
+which is converted to a string by joining the identifiers together
+separated by a single space.</p>
 
 <pre>
 /* regular face of Gentium */
@@ -3039,13 +3040,10 @@ <h3 id="font-variant-position-prop">Subscript and superscript forms: the <a href
 the run, simulated glyphs must be synthesized for all characters using
 reduced forms of the glyphs that would be used without this feature
 applied. This is done per run to avoid a mixture of variant glyphs and
-synthesized ones that would not align correctly. In the case of
-OpenType fonts that lack subscript or superscript glyphs for a given
-character, user agents must use the appropriate subscript and
-superscript metrics specified in the selected font's
-<a href="https://www.microsoft.com/typography/otspec/os2.htm#subxs">OS/2 table</a>
-[[!OPENTYPE]] to calculate the size and offset of the synthesized substitutes.
-</p>
+synthesized ones that would not align correctly. In the case of OpenType
+fonts that lack subscript or superscript glyphs for a given character,
+user agents must synthesize appropriate subscript and superscript
+glyphs.</p>
 
 <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)</p></div>
 
@@ -3061,6 +3059,22 @@ <h3 id="font-variant-position-prop">Subscript and superscript forms: the <a href
 older user agents will still render subscripts and superscripts via
 the older mechanism.</p>
 
+<p>Because <code>font-size: smaller</code> is often used for these
+elements,  the effective scaling factor applied to subscript and
+superscript text varies depending upon the size. For larger text, the
+font size is often reduced by a third but for smaller text sizes, the
+reduction can be much less. This allows subscripts and superscripts to
+remain readable even within elements using small text sizes. User agents
+should consider this when deciding how to synthesize subscript and
+superscript glyphs.</p>
+
+<p class="note">The OpenType font format defines subscript and
+superscript metrics in the
+<a href="https://www.microsoft.com/typography/otspec/os2.htm#subxs">OS/2 table</a>
+[[!OPENTYPE]] but these are not always accurate in practice and so
+cannot be relied upon when synthesizing subscript and superscript
+glyphs.</p>
+
 <p>Authors should note that fonts typically only provide subscript and
 superscript glyphs for a subset of all characters supported by the
 font.  For example, while subscript and superscript glyphs are often
@@ -4596,6 +4610,7 @@ <h3 class="no-num" id="recent-changes">
   <ul>
     <li>add omitted 'font-variant-position' values to <span title="font-variant!!property">'font-variant'</span> shorthand</li>
     <li>make negative values for font-size-adjust invalid, along with negative percentage font-size values</li>
+    <li>remove the requirement that user agents use OS/2 table subscript/superscript metrics</li>
     <li>minor editorial cleanups</li>
   </ul>