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HTML

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <main>
  <h1>Nested animation pausing demo</h1>
  <p><strong>When an animation (tween or timeline) is nested within a timeline, the parent timeline's playhead will continue to run even if the child animation is paused.</strong></p>
  
  <p>The animation below is nested within a parent timeline. If you pause and resume the child timeline, you will likely see a visual jump because the parent timeline's playhead is still running.</p>
  
  <p>In most cases it makes sense to pause the timeline that contains the other animations instead.</p>
  
  <button id="toggle-parent">Pause Parent</button>
  <button id="toggle-child">Pause Child</button>
</main>

<div class="box-wrapper">
  <div class="box"></div>
</div>
<div class="box-wrapper">
  <div class="box"></div>
</div>
<div class="box-wrapper">
  <div class="box"></div>
</div>
<div class="box-wrapper">
  <div class="box"></div>
</div>
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CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Signika+Negative:300,400&display=swap');
body { 
  font-family: "Signika Negative", sans-serif; 
  font-weight: 300;
}

main {
  max-width: 800px;
  margin-bottom: 20px;
}

.box {
  height: 20px;
  background: blue;
  border:2px solid white;
  transform-origin: left center;
}
!

JS 

              
                const childAnimation = gsap.from('.box', {scaleX: 0.01, stagger: 0.5});

const parentTimeline = gsap.timeline({ repeat: -1 });
parentTimeline.add(childAnimation);

const toggleChildButton = document.getElementById("toggle-child");
toggleChildButton.addEventListener("click", function(event) {
  if (event.target.innerHTML === "Pause Child") {
    event.target.innerHTML = "Resume Child";
    childAnimation.pause();
  } else {
    event.target.innerHTML = "Pause Child";
    childAnimation.resume();
  }
});

const toggleParentButton = document.getElementById("toggle-parent");
toggleParentButton.addEventListener("click", function(event) {
  if (event.target.innerHTML === "Pause Parent") {
    event.target.innerHTML = "Resume Parent";
    parentTimeline.pause();
  } else {
    event.target.innerHTML = "Pause Parent";
    parentTimeline.resume();
  }
});
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999px

Console