[css-fonts] tighten the definition of First Available Font

Closes w3c#1765 and w3c#1798

Author: frivoal

css-fonts-3/Fonts.html

@@ -3478,14 +3478,14 @@ <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="https://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).
+  <p>The <dfn id=first-available-font>first available font</dfn>,
+   used for example in the definition of <a href="https://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>’
+   or in the definition of the <a href="https://www.w3.org/TR/CSS2/visudet.html#propdef-line-height">‘<code class=property>line-height</code>’</a> property,
+   is defined to be the first available font
+   that would match the U+0020 (space) 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>
 

css-fonts-3/Fonts.src.html

@@ -2566,11 +2566,14 @@ <h3 id="font-style-matching">Matching font styles</h3>
 as consistent as possible across user agents, given an identical set
 of available fonts and rendering technology.</p>
 
-<p>The <dfn>first available font</dfn>, used in the definition of
-<a href="https://www.w3.org/TR/css3-values/#font-relative-lengths"><em>font-relative lengths</em></a>
-such as 'ex' and 'ch', is defined to be the first available font that
-would match any character given font families in the 'font-family'
-list (or a user agent's default font if none are available).</p>
+<p>The <dfn>first available font</dfn>,
+used for example in the definition of <a href="https://www.w3.org/TR/css3-values/#font-relative-lengths"><em>font-relative lengths</em></a>
+such as 'ex' and 'ch'
+or in the definition of the <a href="https://www.w3.org/TR/CSS2/visudet.html#propdef-line-height">'line-height'</a> property,
+is defined to be the first available font
+that would match the U+0020 (space) character
+given font families in the 'font-family' list
+(or a user agent's default font if none are available).</p>
 
 <h3 id="cluster-matching">Cluster matching</h3>
 

css-fonts-4/Overview.bs

@@ -2610,9 +2610,10 @@ to ensure that the results are as consistent as possible across user agents,
 given an identical set of available fonts and rendering technology.
 
 The <dfn>first available font</dfn>,
-used in the definition of <a>font-relative lengths</a> such as ''ex'' and ''ch'',
+used for example in the definition of <a>font-relative lengths</a> such as ''ex'' and ''ch''
+or in the definition of the 'line-height' property
 is defined to be the first available font
-that would match any character
+that would match the U+0020 (space) character
 given font families in the 'font-family' list
 (or a user agent's default font if none are available).