[css-fonts-4] Export “first available font”, which we rely on in other specs.

Author: fantasai

css-fonts-4/Overview.bs

@@ -2644,7 +2644,7 @@ Matching occurs in a well-defined order
 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>,
+The <dfn export>first available font</dfn>,
 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