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Test Info:

<!DOCTYPE html>
<html class="reftest-wait">
Opacity transition winning over !important rule creates a stacking context
span {
height: 100px;
width: 100px;
position: fixed;
background: green;
top: 50px;
#test {
width: 100px; height: 100px;
background: blue;
* On the compositor we use a previous vsync time stamp rather than the
* current time stamp, as a result sometimes transition may be still in
* before phase after waiting a frame. To compose the same opacity value
* regardless of whether the transition is in before or active phase, we use
* steps(1, end) here.
transition: opacity 100s steps(1, end);
opacity: 1 ! important;
<div id="test"></div>
window.addEventListener("load", () => {
var target = document.getElementById("test");
getComputedStyle(target).opacity;"opacity", "0", "important");