<!DOCTYPE html>
<meta charset="utf-8">
<title>Composition of transform animations</title>
<link rel="help" href="https://drafts.csswg.org/css-transforms-2/#combining-transform-lists">
<meta name="assert" content="transform animations should composite correctly">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<div id="target"><div>

<script>
test(() => {
  var anim1 = target.animate(
      { transform: [ 'translateX(0)', 'translateX(100px)' ]},
      1000
  );
  var anim2 = target.animate(
      { transform: [ 'translateY(0)', 'translateY(100px)' ]},
      {duration: 1000, composite: 'add'}
  );

  anim1.pause();
  anim2.pause();

  anim1.currentTime = 200;
  anim2.currentTime = 800;

  // The computation here should be:
  //   underlying_value = 'translateX(0)' --> 'translateX(100px)' @ 0.2
  //                    = 'translateX(20px)'
  //   final_value = 0.2 * ('translateX(20px) translateY(0)') +
  //                 0.8 * ('translateX(20px) translateY(100px)')
  //               = 'translateX(20px) translateY(80px)'
  //               = 'matrix(1, 0, 0, 1, 20, 80)'
  assert_equals(getComputedStyle(target).transform, 'matrix(1, 0, 0, 1, 20, 80)')
}, 'An additive transform animation on-top of a replace transform animation ' +
    'should composite correctly');
</script>