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CSS

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JavaScript

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HTML 

              
                h1 Меню с плавающей полоской в стиле Lava-Lamp
// измените main-nav на любой класс, не забудьте при этом изменить main-nav в стилях и скриптах
- var c = 'main-nav'

nav(class=c)
	- for (var i = 1; i <= 3; i++)
		if i === 2
			a(href="#", class=c + '__item-parent')
				em(class=c + '__item' + ' ' + c + '__item--on')= 'Item - ' + i
		else if i === 3
			a(href="#", class=c + '__item-parent')
				em(class=c + '__item')= 'Long item - ' + i
		else
			a(href="#", class=c + '__item-parent')
				em(class=c + '__item')= 'Item - ' + i
!

CSS 

              
                @import 'nib'

green = #85C447
blue = #4AC5FF
red = #F14B50

.main-nav
	position relative
	clearfix()
	&, *
		display block
		margin 0
		padding 0
		box-sizing border-box
		font-weight normal
		font-style normal
	&__item-parent
		float left
		padding 10px 20px
	&__lodash
		position absolute
		bottom 0
		height 5px
	&__lodash--on
		background blue
		z-index 99
	&__lodash--hover
		background red
		z-index 9
!

JS 

              
                (function($){
 	jQuery.fn.lavaLampLodash = function(opt){
		opt = $.extend({
			speed: 500, // скорость анимации
			prefix: 'main-nav', // префикс для классов внутри меню
			returnStarting: true // если true, то плавающая полоска при убирании с меню будет улетать к активному пункту
		}, opt);

		var make = function() {
			var $this = $(this);
			var itemParent = $this.find('.' + opt.prefix + '__item-parent');
			var item = $this.find('.' + opt.prefix + '__item');
			var onItem = $this.find('.' + opt.prefix + '__item--on');
			var onLodash = $('<i/>', {
				class: opt.prefix + '__lodash ' + opt.prefix + '__lodash--on'
			});
			var hoverLodash = $('<i/>', {
				class: opt.prefix + '__lodash ' + opt.prefix + '__lodash--hover'
			});
			var setCssLodash = function(el, stepfather) {
				el.css({
					width: stepfather.width(),
					left: stepfather.position().left
				});
			};
			var setAnimateLodash = function(el, stepfather) {
				el.stop().animate({
					width: stepfather.width(),
					left: stepfather.position().left
				}, opt.speed);
			};
			
			$this.append(onLodash, hoverLodash);
			setCssLodash(onLodash, onItem);
			setCssLodash(hoverLodash, onItem);
			
			itemParent.hover(
				function() {
					var $el = $(this).find('.' + opt.prefix + '__item');
					setAnimateLodash(hoverLodash, $el);
				}, function() {
					var $el = opt.returnStarting ? onItem : $(this).find('.' + opt.prefix + '__item');
					setAnimateLodash(hoverLodash, $el);
				}
			);
		};

		return this.each(make); 
	};
})(jQuery);

$('.main-nav').lavaLampLodash({
	speed: 400,
	returnStarting: true,
	prefix: 'main-nav'
});
!
999px

Console