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Adding Classes

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CSS

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HTML 

              
                <nav id="top-menu">
	<div class="container">
		<ul role="menubar">
			<li role="presentation"><a role="menuitem" href="#0">Item 1</a></li>
			<li role="presentation">
				<h2>Item 2</h2>
				<ul>
					<li role="presentation"><a role="menuitem" href="#0">Item 2-1</a></li>
					<li role="presentation"><a role="menuitem" href="#0">Item 2-2</a></li>
				</ul>
			</li>
			<li role="presentation">
				<h2>Item 3</h2>
				<ul>
					<li role="presentation"><a role="menuitem" href="#0">Item 3-1</a></li>
					<li role="presentation"><a role="menuitem" href="#0">Item 3-2</a></li>
					<li role="presentation" class="spacer"></li>
					<li role="presentation">
						<a role="menuitem" href="#0">Item 3-3</a>
						<ul>
							<li role="presentation"><a role="menuitem" href="#0">Item 3-3-1</a></li>
							<li role="presentation"><a role="menuitem" href="#0">Item 3-3-2</a></li>
							<li role="presentation"><a role="menuitem" href="#0">Item 3-3-3</a></li>
						</ul>
					</li>
					<li role="presentation"><a role="menuitem" href="#0">Item 3-4</a> </li>
					<li role="presentation"><a role="menuitem" href="#0">Item 3-5</a></li>
				</ul>
			</li>
			<li role="presentation">
				<h2>Item 4</h2>
				<ul>
					<li role="presentation"><a role="menuitem" href="#0">Item 4-1</a></li>
					<li role="presentation"><a role="menuitem" href="#0">Item 4-2</a></li>
				</ul>
			</li>
			<li role="presentation">
				<h2>Item 5</h2>
				<ul>
					<li role="presentation"><a role="menuitem" href="#0">Item 5-1</a></li>
					<li role="presentation"><a role="menuitem" href="#0">Item 5-2</a></li>
					<li role="presentation">
						<a role="menuitem" href="#0">Item 5-3</a>
						<ul>
							<li role="presentation"><a role="menuitem" href="#0">Item 5-3-1</a></li>
							<li role="presentation"><a role="menuitem" href="#0">Item 5-3-2</a></li>
							<li role="presentation"><a role="menuitem" href="#0">Item 5-3-3</a></li>
						</ul>
            </li>
					<li role="presentation"><a role="menuitem" href="#0">Item 5-4</a></li>
				</ul>
			</li>
		</ul>
	</div>
</nav>
!

CSS 

              
                /**
 * Accessible and keyboard-usable full-CSS multilevel dropdown menu
 */

/* Skip straight to line 95 to see all the magic */

/* Let's define some values */

@color-secondary: #4caf50;
@color-secondary-alternate: #388e3c;

@top-menu-height: 3em;

/* Let's add a little reset and some basic common styles (thanks Eric Meyer!) */

html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed, 
figure, figcaption, footer, header, hgroup, 
main, menu, nav, output, ruby, section, summary,
time, mark, audio, video {
	margin: 0;
	padding: 0;
	border: 0;
	font-size: 100%;
	font: inherit;
	vertical-align: baseline;
	box-sizing: border-box;

	&::before, &::after{
		box-sizing: border-box;
	}
}
article, aside, details, figcaption, figure, 
footer, header, hgroup, main, menu, nav, section {
	display: block;
}
body {
	line-height: 1;
}
ol, ul {
	list-style: none;
}
blockquote, q {
	quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
	content: '';
	content: none;
}
table {
	border-collapse: collapse;
	border-spacing: 0;
}

html{
	font-family: Tahoma, Geneva, Kalimati, sans-serif;
	line-height: 1.5;
	background-color: #f5f5f5;
	-webkit-text-size-adjust: 100%;
	text-rendering: optimizeLegibility;
}

a{
	color: inherit;
	text-decoration: underline;

	&:hover{
		text-decoration: none;
	}
	&:focus{
		text-decoration: none;
	}
	&:active{
		opacity: .8;
	}
}

.container{
	width: 60em;
	margin-left: auto;
	margin-right: auto;
}


/* All the magic starts here! */

#top-menu{
	position: relative;
	z-index: 999;
	height: @top-menu-height;
	background-color: @color-secondary;
	color: #fff;

	a, h2{
		position: relative;
		z-index: 5;
		display: block;
		padding: .5em;
		margin: 0;
		line-height: 2;
		font-size: inherit;
		font-weight: inherit;
		text-decoration: none;
		text-align: center;
		white-space: nowrap;
		background-color: @color-secondary;
	}
	a:hover, a:focus{
		background-color: @color-secondary-alternate;
	}
	h2{
		cursor: default;
	}
	
	ul{
		margin: 0;
		list-style: none;

		&>li{
			margin: 0;

			&.spacer{
				height: 1em;
			}
		}
	}

	&>.container>ul{
		&::after{
			&:extend(.pseudo-clear);
		}
		&>li{
			float: left;
			width: 20%;
			overflow: hidden;
			height: @top-menu-height;

			&:nth-child(1){
				width: 10%;
			}
			&:nth-child(3){
				width: 30%;
			}

			&>a:not(:last-child), &>h2:not(:last-child){
				// padding-right: @top-menu-height;

				&::after{
					content: '﹀';
					position: absolute;
					z-index: 6;
					top: 0;
					right: 0;
					bottom: 0;
					padding: .5em;
					width: @top-menu-height;
					height: @top-menu-height;
					font-weight: bold;
					text-align: center;
					line-height: @top-menu-height - .5em;
				}
			}

			&>ul{
				position: relative;
				z-index: 1;
				background-color: @color-secondary;
				box-shadow: 0 0 .5em rgba(0,0,0,.35);

				&>li{
					position: relative;
					overflow: hidden;
					border-top: 1px solid @color-secondary-alternate;

					&>a:not(:last-child){
						// padding-right: @top-menu-height;

						&::after{
							content: '⟩';
							position: absolute;
							z-index: 6;
							top: 0;
							right: 0;
							bottom: 0;
							padding: .5em;
							width: @top-menu-height;
							height: @top-menu-height;
							font-weight: bold;
							text-align: center;
							line-height: @top-menu-height - 1em;
						}
					}

					&>ul{
						position: absolute;
						top: 0;
						left: 100%;
						min-width: 100%;
						z-index: 0;
						background-color: @color-secondary;
						box-shadow: 0 0 .5em rgba(0,0,0,.35);
						border-left: 1px solid @color-secondary-alternate;
						
						&>li:not(:first-child){
							border-top: 1px solid @color-secondary-alternate;
						}
					}
					
					&:hover, &.open{
						overflow: visible;
					}
				}
			}
			
			&:hover, &.open{
				overflow: visible;
			}
		}
    /* Handling the right-side sub-menus to show sub-elements on the left instead of right */
		&>li:last-child>ul>li{
			&>a:not(:last-child){
				// padding-right: .5em;
				// padding-left: @top-menu-height;

				&::after{
					content: '⟨';
					right: auto;
					left: 0;
				}
			}

			&>ul{
				// left: -100%;
				left: auto;
				right: 100%;
				border-right: 1px solid @color-secondary-alternate;
				border-left: none;
			}
		}
	}
}

/* There are only 3 levels for now, but you can see how it wors and adapt it for more if necessary (but is it really?) */
!

JS 

              
                /* The menu is totally usable without JavaScript, but the few lines make it more beautiful */

var topMenu = document.getElementById('top-menu');

function topMenuOpen (e) {
	this.classList.add('open');

	var elem = this;
	while (elem.parentNode !== topMenu) {
		if (elem.nodeName.toLowerCase() == 'li'){
			elem.classList.add('open');
		}
		elem = elem.parentNode;
	}
	if (e && e.target && e.target.nodeName.toLowerCase() === 'a') {
		e.preventDefault();
	}
}
function topMenuClose (e) {
	this.classList.remove('open');

	var elem = this;
	while (elem.parentNode !== topMenu) {
		if (elem.nodeName.toLowerCase() == 'li'){
			elem.classList.remove('open');
		}
		elem = elem.parentNode;
	}

	if (e && e.target && e.target.nodeName.toLowerCase() === 'a') {
		e.preventDefault();
	}
}



Array.prototype.forEach.call(topMenu.querySelectorAll('ul>li'), function (li) {
	if(!li.getElementsByTagName('ul')) {
return;
	}

	li.addEventListener('mouseleave', topMenuClose.bind(li), false);
	li.addEventListener('mouseenter', topMenuOpen.bind(li), false);
});

Array.prototype.forEach.call(topMenu.querySelectorAll('li>a, li>h3'), function (link) {
	if(!link.parentNode.getElementsByTagName('ul')) {
		return;
	}

	link.addEventListener('blur', topMenuClose.bind(link.parentNode), false);
	link.addEventListener('focus', topMenuOpen.bind(link.parentNode), false);
});
!
999px

Console