.htaccess load remote resources if not found locally

gistfile1.txt

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{REQUEST_FILENAME} !-f [NC]
  RewriteRule ^(.*\.(js|css|png|jpe?g|gif|ico)) https://domainname.com/$1 [QSA,L,C]
</IfModule>