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Test Info:

<!DOCTYPE html>
<html class="reftest-wait">
Opacity transition and animation overridden by !important rule creates a
stacking context
span {
height: 100px;
width: 100px;
position: fixed;
background: green;
top: 50px;
@keyframes Opacity0 {
from, to { opacity: 0; }
#test {
width: 100px; height: 100px;
background: blue;
opacity: 0 ! important;
transition: opacity 100s steps(1, start);
animation: Opacity0 100s;
<div id="test"></div>
window.addEventListener("load", () => {
var target = document.getElementById("test");
// Change the opacity style to 100%."opacity", "1", "important");
// Important style is changed but there is a CSS animation,
// so transition won't be visible and the CSS animation is overridden by
// the !important rule. As a result opacity style here should be 100%
// specified by the important rule, but we should create a stacking
// context for it because there are animations.