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HTML

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                nav
	ul
		li
			a(href='#one').active One
		li
			a(href='#two') Two
		li
			a(href='#three') Three
		li
			a(href='#four') Four
		li
			a(href='#five') Five
			
section#one
	p Slide One

section#two
	p Slide Two

section#three
	p Slide Three
	
section#four
	p Slide Four
	
section#five
	p Slide Five
!

CSS 

              
                nav
	position fixed
	
	width 100%
	top 0
	left 0
	right 0
	
	background-color white
	
	ul
		list-style-type none
		display flex
		align-items center
		justify-content space-around
		
		width 100%
		max-width 800px
		height 50px
		
		margin 0 auto
		padding 0
		
	li
		display inline-block

		padding 5px
		
	a
		display block

		height 40px
		
		padding 0 20px
		
		line-height 40px
		text-decoration none
		text-transform uppercase
		color #323232
		font-weight bold
		
		border-radius 4px
		
		transition background-color 0.3s ease-in
		
		&:hover
		&:active
		&:focus
			background-color rgba(#B8D6A8, 0.5)
		
		&.active
			background-color rgba(#B8D6A8, 0.5)
		
		
section
	display flex
	align-items center
	justify-content center
	
	min-height 100vh
	
p
	text-align center
	color white
	font-size 3.5em
	font-weight bold
	text-transform uppercase
	
#one
	background-color #6CA392
	
#two
	background-color #FFA58C
	
#three
	background-color #FF4F30
	
#four
	background-color #576B51
	
#five
	background-color #392A1B
!

JS 

              
                // simple function to use for callback in the intersection observer
const changeNav = (entries, observer) => {
	entries.forEach((entry) => {
		// verify the element is intersecting
		if(entry.isIntersecting && entry.intersectionRatio >= 0.55) {
			// remove old active class
			document.querySelector('.active').classList.remove('active');
			// get id of the intersecting section
			var id = entry.target.getAttribute('id');
			// find matching link & add appropriate class
			var newLink = document.querySelector(`[href="#${id}"]`).classList.add('active');
		}
	});
}

// init the observer
const options = {
	threshold: 0.55
}

const observer = new IntersectionObserver(changeNav, options);

// target the elements to be observed
const sections = document.querySelectorAll('section');
sections.forEach((section) => {
	observer.observe(section);
});
!
999px

Console