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Test Info:

<!DOCTYPE html>
<html class="reftest-wait">
Transform transition and animation overridden by !important rule create
a stacking context
span {
height: 100px;
width: 100px;
position: fixed;
background: green;
top: 50px;
@keyframes Transform100px {
from, to { transform: translateX(100px); }
#test {
width: 100px; height: 100px;
background: blue;
transform: translateX(200px) ! important;
transition: transform 100s steps(1, start);
animation: Transform100px 100s;
<div id="test"></div>
window.addEventListener("load", () => {
var target = document.getElementById("test");
// Change the transform style to 'none'."transform", "none", "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 transform style here should be 'none'
// specified by the important rule, but we should create a stacking
// context for it because there are animations.