base URL for external stylesheet ?

If you don't want to create a sub-domain, you should leave your assets folder alongside you index.php and create a rewrite rule for it. But that means you need to edit your main domains .htaccess file. Also, that means every request are doubled, for CSS, images and JS.

https://stackoverflow.com/a/19494604

Code:

RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(?:[^/]*/)*([^/.]+\.(?:jpe?g|gif|bmp|png|tiff|css|js))$ /assets/$1 [R=301,L,NC]