RewriteRule index.php?url=$1 [QSA, L]
I think you have a couple of glaring typos(?) in your question that make this directive completely invalid... you are missing a RewriteRule pattern, so this won't actually match anything and the erroneous space in the flags argument is syntactically invalid, resulting in a 500 Internal Server Error response?!
The RewriteRule directive should be written like:
RewriteRule ^([\w/-]+)$ index.php?url=$1 [QSA,L]
^([\w/-]+)$ matches a URL-path containing any of the characters: a-z, A-Z, 0-9, _ (underscore), / (slash) and - (hyphen). I've excluded the dot, which is naturally part of real filenames.
You should also ensure that MultiViews is disabled, so that mod_negotiation doesn't rewrite the request before mod_rwrite - since your "extensionless" requests appear to map directly to filenames. eg. /user maps to /user.php. Place the following at the top of your .htaccess file:
Options -MultiViews
But when I was given a slash to my URL it stops showing CSS or other IMG directories.
This isn't a problem with .htaccess, but is caused by using relative client-side URLs to your static resources. When you request the URL /user/anonymous then the browser will resolve any relative URLs relative to /user/anonymous (not the document root - which is probably what you are expecting). If you have a relative URL to css/styles.css then the browser is naturally going to resolve this to /user/css/styles.css - which probably doesn't exist (and is likely getting rewritten to index.php - but that isn't the issue - the fact that it doesn't exist is the issue).
If you look at the network traffic (HTTP requests) in the browser, it should give you a clue as to what's going on.
You need to change your client-side URLs to use either root-relative (starting with a slash) or absolute (scheme + hostname) URLs to fix this issue.
See my answer to the following question on the Webmasters stack that explains this further: