Trigger a CSS Animation when the user scrolls to page section - html

Question

I have a simple CSS animation on my site, where I want to show 5 divs showing one at a time in a row.
Everything works fine, but I want to make a trigger to that animation, when the user scrolls to that particular section on my site(now the animation starts when the page loads).
Here is my code:
<div id="space"></div>
<div id="container">
<img src="https://cdn1.iconfinder.com/data/icons/user-pictures/100/male3-64.png" />
<img src="https://cdn1.iconfinder.com/data/icons/user-pictures/100/male3-64.png" />
<img src="https://cdn1.iconfinder.com/data/icons/user-pictures/100/male3-64.png" />
<img src="https://cdn1.iconfinder.com/data/icons/user-pictures/100/male3-64.png" />
<img src="https://cdn1.iconfinder.com/data/icons/user-pictures/100/male3-64.png" />
</div>
CSS:
#space {
height: 700px;
background-color: blue;
}
#container img {
opacity: 0;
}
#keyframes fdsseq {
100% { opacity: 1; }
}
#container img {
animation: fdsseq .5s forwards;
}
#container img:nth-child(1) {
animation-delay: .5s;
}
#container img:nth-child(2) {
animation-delay: 1s;
}
#container img:nth-child(3) {
animation-delay: 1.5s;
}
#container img:nth-child(4) {
animation-delay: 2s;
}
#container img:nth-child(5) {
animation-delay: 2.5s;
}
https://jsfiddle.net/Lwb088x5/

Answer 1

You need JavaScript to do this.
In the example(s) below, a scroll event listener to attached, and the animate class is added to the #container element if the img elements are visible:
Updated Example
#container.animate img {
animation: animation .5s forwards;
}
document.addEventListener('scroll', function (e) {
var top = window.pageYOffset + window.innerHeight,
isVisible = top > document.querySelector('#container > img').offsetTop;
if (isVisible) {
document.getElementById('container').classList.add('animate');
}
});
Alternatively, you could also use jQuery as well:
Updated Example
$(window).on('scroll', function (e) {
var top = $(window).scrollTop() + $(window).height(),
isVisible = top > $('#container img').offset().top;
$('#container').toggleClass('animate', isVisible);
});