Bug 248145

We're failing a couple of tests in web-animations/animation-model/keyframe-effects/effect-value-context-filling.html because we fail to update keyframes when a used CSS variable changes. For instance:

test(t => {
  const div = createDiv(t);
  div.style.setProperty('--target', '100px');
  const animation = div.animate(
    [{ marginLeft: '0px' }, { marginLeft: 'var(--target)' }],
    { duration: 1000, fill: 'forwards' }
  );
  animation.finish();
  assert_equals(
    getComputedStyle(div).marginLeft,
    '100px',
    'Effect value before updating variable'
  );

  div.style.setProperty('--target', '200px');

  assert_equals(
    getComputedStyle(div).marginLeft,
    '200px',
    'Effect value after updating variable'
  );
}, 'Filling effect values reflect changes to variables on element');

In fact, after fixing bug 186490, the last two remaining failures in that test are due to this issue.