<!DOCTYPE html>
<title>Test that non-transitioned style is responsive to transitioning properties</title>
<link rel="help" href="https://drafts.csswg.org/css-transitions/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #target1 {
    transition: font-size steps(2, start) 1000s;
    font-size: 10px;
    width: 1em;
  }
  #target1.change {
    font-size: 20px;
  }
  #ref1 {
    width: 15px;
  }

  #target2 {
    transition: font-size steps(2, start) 1000s;
    font-size: 10px;
    width: 1ex;
  }
  #target2.change {
    font-size: 20px;
  }
  #ref2 {
    font-size: 15px;
    width: 1ex;
  }

  #target3 {
    transition: --x steps(2, start) 1000s;
    --x: 10px;
    width: var(--x);
  }
  #target3.change {
    --x: 20px;
    font-size: 20px;
  }
  #ref3 {
    width: 20px;
  }

</style>
<div id="targets">
  <div id="target1"></div>
  <div id="target2"></div>
  <div id="target3"></div>
</div>
<div id="refs">
  <div id="ref1"></div>
  <div id="ref2"></div>
  <div id="ref3"></div>
</div>
<script>

// Test that the computed value of the given property is equal on
// 'target' and 'ref', after applying the transition to 'target'.
function test_ref(target, ref, property, description) {
  test(() => {
    let unused = getComputedStyle(target).getPropertyValue(property);
    target.className = 'change';
    let actual = getComputedStyle(target).getPropertyValue(property);
    let expected = getComputedStyle(ref).getPropertyValue(property);
    assert_equals(actual, expected);
  }, description);
}

test_ref(target1, ref1, 'width', 'em units respond to font-size transition');
test_ref(target2, ref2, 'width', 'ex units respond to font-size transition');
test_ref(target3, ref3, 'width', 'var() references respond to custom property transition');

</script>