We fail this subtest in web-animations/interfaces/Animation/commitStyles.html:

test(t => {
  const div = createDiv(t);
  div.style.fontSize = '10px';

  const animation = div.animate(
    { lineHeight: '1.5' },
    { duration: 1, fill: 'forwards' }
  );
  animation.finish();
  animation.commitStyles();
  animation.cancel();

  assert_numeric_style_equals(getComputedStyle(div).lineHeight, 15);
  assert_equals(div.style.lineHeight, "1.5", "line-height is committed as a relative value");

  div.style.fontSize = '20px';
  assert_numeric_style_equals(getComputedStyle(div).lineHeight, 30,
      "Changes to the font-size should affect the committed line-height");

}, 'Commits relative line-height');

This is because of BuilderConverter::convertLineHeight() where a line-height specified as a percentage is stored as a LengthType::Fixed, while a line-height specified as number is stored as LengthType::Percent.

We'll somehow need to account for `specifiedLineHeight()`.