[css3-fonts] define first available font

Author: John Daggett

css-fonts/Fonts.html

@@ -1653,6 +1653,13 @@ <h3 id=font-size-adjust-prop><span class=secno>3.6 </span>Relative sizing:
    id=aspect-value>aspect value</dfn></a>. Precisely defined, it is equal to
    the x-height of a font divided by the font size.
 
+  <p class=issue>How does the use of ‘<a
+   href="#propdef-font-size-adjust"><code
+   class=property>font-size-adjust</code></a>’ affect <a
+   href="http://www.w3.org/TR/css3-values/#font-relative-lengths"><em>font-relative</em></a>
+   unit sizes? Are all units affected the same way, i.e. including the
+   definition of ‘<code class=property>em</code>’?
+
   <p>In situations where font fallback occurs, fallback fonts may not share
    the same aspect ratio as the desired font family and will thus appear less
    readable. The font-size-adjust property is a way to preserve the
@@ -3012,8 +3019,8 @@ <h3 id=font-face-loading><span class=secno>4.8 </span>Font loading
    fallback font. In cases where the font download fails user agents must
    display text, simply leaving transparent text is considered non-conformant
    behavior. Authors are advised to use fallback fonts in their font lists
-   that closely match the vertical metrics of the downloadable fonts to avoid
-   large page reflows where possible.
+   that closely match the metrics of the downloadable fonts to avoid large
+   page reflows where possible.
 
   <h3 id=same-origin-restriction><span class=secno>4.9 </span>Same-origin
    restriction for fonts</h3>
@@ -3348,6 +3355,15 @@ <h3 id=font-style-matching><span class=secno>5.2 </span>Matching font
    consistent as possible across user agents, given an identical set of
    available fonts and rendering technology.
 
+  <p>The <dfn id=first-available-font>first available font</dfn>, used in the
+   definition of <a
+   href="http://www.w3.org/TR/css3-values/#font-relative-lengths"><em>font-relative
+   lengths</em></a> such as ‘<code class=property>ex</code>’ and ‘<code
+   class=property>ch</code>’, is defined to be the first available font
+   that would match any character given font families in the ‘<code
+   class=property>font-family</code>’ list (or a user agent's default font
+   if none are available).
+
   <h3 id=cluster-matching><span class=secno>5.3 </span>Cluster matching</h3>
 
   <p>When text contains characters such as combining marks, ideally the base
@@ -6472,6 +6488,9 @@ <h2 class=no-num id=index>Index</h2>
    <li>feature_value_definition, <a href="#featurevaluedefinition"
     title="feature_value_definition"><strong>6.9.1</strong></a>
 
+   <li>first available font, <a href="#first-available-font" title="first
+    available font"><strong>5.2</strong></a>
+
    <li>font, <a href="#propdef-font" title=font><strong>3.7</strong></a>
 
    <li>font specific, <a href="#font-specific" title="font