[css-fonts-4] Fix markup w3c#9969

Author: cdoublev

css-fonts-4/Overview.bs

@@ -7343,7 +7343,7 @@ Animation type: by computed value
 	parsing/font-palette-valid.html
 </wpt>
 
-With the <code>palette-mix()</code> function defined as follows:
+With the <dfn>palette-mix()</dfn> function defined as follows:
 
 <pre class="prod def" nohighlight>
 <l><<palette-mix()>></l> = palette-mix(<<color-interpolation-method>> , [ [normal | light | dark | <<palette-identifier>> | <<palette-mix()>> ] && <<percentage [0,100]>>? ]#{2})