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HTML

              
                <html>
  <body>

    <div class="intro" style="color:white">
      <h1>Scroll Down</h1>
    </div>
      
        <div class="inview-element">
      <h2>I've been scrolled to</h2>
    </div>
    
        <div class="inview-element">
      <h2>I've been scrolled to</h2>
    </div>
    
        <div class="inview-element">
      <h2>I've been scrolled to</h2>
    </div>
    
            <div class="inview-element">
      <h2>I've been scrolled to</h2>
    </div>
    
  </body>
</html>
              
            
!

CSS

              
                body {
  font-family: sans-serif;
}

.inview-element, .intro {
  min-height: 100vh;
  background-color: darkblue;
  display:flex;
  align-items: center;
  justify-content: center;
}

.inview-element {
    transition:2s;
}
              
            
!

JS

              
                let observedElements = document.querySelectorAll('.inview-element'); // Define the elements you want to intiate an action on

const options = { //define your options
  threshold: 0.5
}

const inViewCallback = entries => {
  entries.forEach(entry => {
    if (entry.isIntersecting) { // define the event/property you want to use
      //do something with the element, in our case, add a class 
      entry.target.classList.add('inview');  
      entry.target.style.backgroundColor ="beige";
   }
    else { 
      // OPTIONAL, in case you want to do something once the intersection is done
   }
  });
}

let observer = new IntersectionObserver(inViewCallback,options); // create a new instance using our callback which contains our elements and actions, using the options we defined

observedElements.forEach(element => {
  observer.observe(element) // run the observer 
});
              
            
!
999px

Console