Serve static assets with an efficient cache policy

This is not related to HTML pages or Supercache. It means your static assets – stylesheet files (.css) and javascript files (.js) don’t have efficient cache policy.
If you are on Apache server and know how to edit .htaccess, add this to it for a 1 month expiry:

# Cache Control and Enable CORS
<FilesMatch "\.(js|css|jpg|gif|png|pdf|swf|svg|svgz|ico|ttf|ttc|otf|eot|woff|woff2|webp)$">
 <IfModule mod_headers.c>
    ExpiresActive On
    ExpiresDefault  "access plus 1 month"
    Header set Cache-Control "public, immutable, max-age=2628000, s-maxage=2628000"
    Header set Access-Control-Allow-Origin "*"
 </IfModule>
</FilesMatch>

Alternately, install HTTP Headers plugin, and go to the section “Cache Control” and/or “Expiry”, and enter similar options to those appearing above. Make sure you are using Supercache in “Simple” (Recommended) mode or this won’t work.
I haven’t used that plugin for ages, so just check it is applying the cache control to CSS and JS, not just HTML…