<div class="container">
    <div class="child child-1">Child 1 (z-index: 2)</div>
    <div class="child child-2">Child 2 (z-index: 1)</div>
</div>
<div class="sibling">Sibling (z-index: 1)</div>

.container {
    container-type: inline-size;
    width: 300px;
    height: 300px;
    border: 2px solid black;
    padding: 1rem;
    margin: 2rem;
    contain: layout; /* this restores the rendering to how it was prior to Chrome 129 */
}

.child, .sibling {
    position: absolute;
    display: flex;
    justify-content: center;
    align-items: center;
    color: white;
}

.child {
    width: 200px;
    height: 200px;
}

.child-1 {
    background-color: rgba(255, 0, 0, 0.7);
    top: 100px;
    left: 100px;
    z-index: 2;
}

.child-2 {
    background-color: rgba(0, 255, 0, 0.7);
    top: 30px;
    left: 0;
    z-index: 1;
}

.sibling {
    width: 150px;
    height: 150px;
    background-color: rgba(0, 0, 255, 0.7);
    top: 165px;
    left: 50px;
    z-index: 1;
}