#mainnav {
    
}

    #mainnav ul {
        margin: 0;
		padding: 0;
        list-style-type: none;
    }
    
    #mainnav li {
        float: left;
        display: inline;
        position: relative;
        z-index: 15;
    }
    
        #mainnav li a {
            display: block;
            font-size: 16px;
			line-height: 1em;
            text-decoration: none;
            padding: 12px;
        }
        
        #mainnav .parent ul {
        	position: absolute;
        	z-index: 16;
        	top: 40px;
        	left: 0;
			display: none;
        }
        
        	#mainnav .parent li {
        		float: none;
        		display: block;
				white-space: nowrap;
        	}
        	
        		#mainnav .parent li a {
        			font-size: 14px;
        			font-weight: normal;
        			padding: 8px;
        		}

Basic style to set up Joomla-format menu as horizontal menu with dropdowns.

Horizontal Menu w/ Dropdowns

dropdown, joomla

CSS