fonts-conf(5) fonts.conf example still uses "sans" instead of "sans-serif".
quotation:
. . .
<prefer>
<family>Arial</family>
</prefer>
<default>
<family>sans</family> <-------------
</default>
</alias>
<alias>
<family>Courier</family>
<prefer>
<family>Courier New</family>
</prefer>
<default>
<family>monospace</family>
</default>
</alias>
<!--
Provide required aliases for standard names
Do these after the users configuration file so that
any aliases there are used preferentially
-->
<alias>
<family>serif</family>
<prefer>
<family>Times New Roman</family>
</prefer>
</alias>
<alias>
<family>sans</family> <-------------
<prefer>
<family>Arial</family>
</prefer>
</alias>
<alias>
<family>monospace</family>
<prefer>
. . .