my web does not show styles using friendly urls with htaccess

I'm making my page and I use htaccess to create friendly urls, in it, I tell an item to do a href to a "contact" file

html:
 <a href="contacto/">

.htaccess
Rewriterule  contacto/                                 contacto.html

and the address works well, but css styles are not displayed properly.

but if I enter the page with the name of the file.php or html if you take them. Does anyone know and can explain why this happens and how to solve it using htacces?