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<!doctype html>
<html>
<head>
<meta charset=utf-8>
<title>CSS Transitions Test: Changing transition properties mid-transition</title>
<meta name="assert" content="When transition properties are changed mid-transition, the original transition completes and new transition parameters apply to subsequent changes">
<link rel="help" href="https://drafts.csswg.org/css-transitions/#starting">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./support/helper.js"></script>
</head>
<body>
<div id="log"></div>
<script>
promise_test(async t => {
const div = addDiv(t, {
style: 'transition: width 100ms; width: 0px;'
});
// Flush initial state
getComputedStyle(div).width;
// Start first transition to 100px
div.style.width = '100px';
// Wait until transition starts running
await new Promise(resolve => {
div.addEventListener('transitionrun', resolve, { once: true });
});
// MID-TRANSITION CHANGE: Switch to 0s duration + 100ms delay
div.style.transition = 'width 0s 100ms';
assert_not_equals(
getComputedStyle(div).width,
'100px',
'Width should not reach 100px yet'
);
// Trigger new width change mid-transition
div.style.width = '0px';
assert_not_equals(
getComputedStyle(div).width,
'0px',
'Width should not changed to 0px immediately'
);
await waitForTransitionEnd(div);
// Final computed style
assert_equals(
getComputedStyle(div).width,
'0px',
'Final width should be 0px'
);
}, 'Mid-transition transition changes affect subsequent transitions');
</script>
</body>
</html>
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