PHP .htaccess用户名重定向

I have dilemma for my .htaccess:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([a-zA-Z0-9_-]+)$ user.php?user=$1

Example:

When I open http://example.com/david it opened as well.

But if I add slash after username, it won't open, like http://example.com/david/ (note the trailing slash).

Have you tried

RewriteRule ^([a-zA-Z0-9_-]+)/$ user.php?user=$1

You can use:

RewriteRule ^([a-zA-Z0-9_-]+)/?$ user.php?user=$1

And for css:

css disapears when trying to create clean url's

(Do not accept a solution if it does not work!)